I Tried The K18 Leave In Molecular Repair Mask, And It Made Me A Believer

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Written by Annie Walton-Doyle

Updated: March 15, 2024

We purchased the K18 Leave In Molecular Repair Mask for our writer to put to the test. Keep reading for our full, hands-on product review.

K18 Mask 700 x 400
Annie Walton-Doyle / Timeless Hairstyles

The Bottom Line

For anyone who wants a product that actually repairs hair damage, the K18 Leave In Molecular Repair Mask is absolutely worth looking into. While you may be sceptical over its efficacy on your first use, the results speak for themselves. While it won’t replace your usual conditioner or hair mask, it can be a worthwhile addition to your hair care routine, particularly if you have damaged or colour-treated locks.

If you’ve even had a perfunctory glance at TikTok’s hair influencers’ feeds over the past few months, you’ll have heard all about the K18 Leave In Molecular Repair Mask. Touted as the next Olaplex, this is a hair treatment unlike any other.

The brand’s patented polypeptide (the K18 of its name) works to actually repair individual strands of hair, undoing damage for a healthier, smoother, and sleeker look. If you’ve ever used to conventional, thick and buttery, rinse-out hair masks, the K18 Leave In Molecular Repair Mask signifies quite the departure – but offers some surprising results. 

Read on for my full review.

What We Like

  • Actually repairs the hair, rather than just concealing damage
  • Minimises frizz and flyaways
  • Offers heat protection

What We Don’t Like

  • Usage takes some getting used to
  • Higher price point

The Facts

  • K18 Leave In Molecular Repair Mask
  • Best For: Dry or damaged hair
  • Can Be Used For: Straight, wavy, curly, and coily hair types
  • Cruelty Free? Yes
  • Key Ingredients: sh-Oligopeptide-78 (K18Peptide™), Hydrolyzed Wheat Protein
  • Price: $75, though a trial size is available for $29
  • Size: 1.7 fluid ounces (or 0.5 fluid ounces for the mini)

About The Brand

K18 is a science-first hair brand, that uses technological advances to actually repair hair damage. The brand is named after its patented K18 peptide, which repairs damaged hair strands from the inside out. The K18 peptide fills in the gaps in the polypeptide chains which make up strands of hair, re-bonding the strand. It improves the condition of hair, not just the look of it.

The most famous and popular product from the K18 line is the Leave-In Molecular Repair Hair Mask, which is, as the name suggests, a leave-in mask that can be used in the place of a conditioner. The brand also offers shampoos and the K18 Molecular Repair Hair Oil.

About My Hair: Long, Damaged, and Sometimes Dry

K18 Mask Long
Annie Walton-Doyle / Timeless Hairstyles

Like (pretty much) any woman out there, my hair has been through it. I box-dye my hair red at home, which obviously comes with some damage.

I also swim in a chlorinated pool as my main form of exercise, which I find similarly drying and damaging to my locks. Because of all this, any product which promises to help undo hair damage automatically piques my interest.

My hair routine is already fairly tailored around the aims of minimizing damage, while adding hydration and nourishment. I wash with a gentle but deeply cleansing shampoo to properly combat chlorine, then use either a hydrating conditioner or a hair mask to pack in as much moisture as possible. I finish with a leave-in product of oil to help my hair air-dry as smoothly, softly, and free from frizz as possible. 

The K18 Leave In Mask has been on my radar for years, and I was eager to test out the much-hyped treatment.

Anything that uses actual science that I don’t even properly understand must work, right? With claims such as restoring 91% of hair’s original strength and 94% of its original elasticity, I couldn’t wait to see the results for myself.

Mask Feel: Lightweight and Unassuming

Upon first pump of the K18 Leave In Molecular Repair Mask, I was a little concerned. This thin and barely there fluid didn’t seem like something that could properly coat and cover my hair, and I just didn’t see how it could possibly live up to its claims – or its hype!

The product feels like a soft and bouncy gel and is a far cry from the typical thick and unctuous masks I usually reach for. For this reason, I was concerned that my hair would feel brittle and dry after use, almost like I had skipped my conditioning step entirely. However, to give the game away slightly, this was not the case at all…

How To Use The K18 Mask

K18 recommends using the mask in a very particular way.

You first shampoo your hair, then towel dry it and apply just three pumps of the mask. Once you’ve worked the mask through your hair, you wait for four minutes before combing it through and styling it as usual, without rinsing it out. 

As someone who’s been using hair masks for years, this whole process went against my natural instincts. However, it really did work.

After washing, towel-drying, and applying the mask, my comb glided through my hair. I never heat-style my hair, as it has a pretty easygoing naturally straight texture, so I finished with the K18 Molecular Repair Hair Oil and allowed my mane to dry naturally.

I have to say, after my initial scepticism, I was blown away at how soft and smooth my hair could feel, without the use of a ‘normal’ conditioner.

The Results: Repaired Strands With A Touchably Soft Feel

K18 Mask Long 2
Annie Walton-Doyle / Timeless Hairstyles

After using the K18 mask just once, I  immediately noticed that my hair felt pleasantly smooth and soft.

The brand recommends you use the mask for at least four consecutive washes to achieve maximum results, then keep it in your rotation for every fourth or so wash to maintain the reparative qualities. The improved condition of my hair just gets more extreme over time, and having this mask in my arsenal for times of need offers some real reassurance about any potential future damage!

The Ingredients: The Science Bit

The entire K18 line is highly ingredients-focused, and the K18 mask is no exception. The range is built around its patented peptide, which offers some pretty impressive claims like 24 hours of frizz control and a 78% reduction in split ends. Alongside this peptide, the mask also contains wheat protein, which helps to build up hair strength.

The Value: Pricey – But Worth It

Let’s face it: the K18 product range has a high-end price point. But I really do feel as though it’s a case of you getting what you pay for, especially when it comes to the K18 mask.

Anyone who colors, heat-treats, bleaches, or otherwise inflicts damage on their locks could benefit from having this treatment on rotation. I can’t say much more than it really does work, and if soft, strong hair is important to you, you will not be disappointed in this purchase.

Similar Products: Reparative Hair Treatments

The Olaplex No. 3 Hair Perfector is the closest product on the market (that I’ve tried) to the K18 mask. Both ranges are designed to treat weakened and brittle hair on a strand-by-strand level, repairing damage to boost strength.

However, after using both, I must admit I’m a K18 convert. The results were more instantaneously impressive, plus over time, I noticed a more visible improvement in the condition of my strands. Nothing else on the market quite compares!

The Final Verdict

For anyone looking to undo damage to their locks, the K18 Leave In Molecular Repair Mask is the most impressive at-home treatment I’ve come across.

While the lightweight feel and bizarre method of usage may feel counter-intuitive, after following the instructions just once, I noticed a real improvement in my hair’s condition. And what’s even better is that over time, I can only expect more and more improvement!

The Specifications

  • Brand: K18 Haircare
  • Product Name: K18 Leave In Molecular Repair Mask
  • Price: $29 – $75 each depending on which size you go for.
  • Size: 0.5 fluid ounces – 1.7 fluid ounce

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