K18 vs. Olaplex: Which Leave-In Hair Treatment Comes Out on Top?

Between all the blow-drying and other heat styling we do on a regular basis, and the ongoing color upkeep for highlights, our hair is constantly put through the wringer. That means we’re often combatting split ends, dryness and frizz to rival Albert Einstein. Sure, you could haul yourself back to the salon for a keratin treatment, but not everyone has two hours and $300 to spare several times a year. That’s where at-home hair repair treatments come in handy. Here, PureWow’s Editor-in-Chief, Jillian Quint, tests two popular products—K18’s Leave-In Molecular Repair Hair Mask vs Olaplex’s No. 3 Hair Repair Perfector—to see which one comes out on top. Read on for an overview of what differentiates the brands and their approach to hair repair, plus Quint’s experience using both products over a four-week period.

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K18 Biomimetic Hair Science is a haircare brand that, as its name implies, creates treatments that mimic nature. Specifically, the K18 bioactive peptide is designed to imitate keratin’s structure and repair broken amino acid (polypeptide) bonds. K18’s patented “K18PEPTIDE” is the foundation of its entire product line, which includes the viral molecular repair hair mask, hair oil, shampoo and conditioner, promising results in as little as four minutes.

Olaplex offers an extensive lineup of haircare products, including shampoo, stylers, reparative treatments and even brow and lash products. The star ingredient is the bis-aminopropyl diglycol dimaleate molecule, which focuses on repairing the hair’s disulfide bonds. Broken disulfide bonds can lead to the presence of cysteic acid, which eats away at the hair shaft. Olaplex treatments bind to the sulfur hydrogen molecule before it can attach to oxygen and become cysteic acid.

The main difference between K18 and Olaplex are their approaches to hair repair. K18 focuses on polypeptide bonds (aka keratin, which forms the actual structure of the hair), while Olaplex treats disulfide bonds, which keep the polypeptides lined up.

PureWow’s Editor-in-Chief, Jillian Quint, has used Olaplex for years. She recently tested K18’s molecular repair hair mask for two weeks (four washes) to see how the two brands stacked up against each other.

Quint has thick, dry and frizzy hair that she says has “been colored within an inch of its life.” Because she has dyed her hair consistently for the past seven years, it requires weekly repair treatments.

To test the products, she applied both as directed, then followed up with a hydrating serum and heat protectant before blow-drying. She also uses purple shampoo and conditioner once a month to counter brassy tones in her highlights.  

“Though technically not a conditioner, it essentially functions as one,” Quint notes. “I think of this guy as the best leave-in-treatment on the market.”

Not everyone has the time to luxuriate for an hour (or even twenty minutes) while a product does its thing. Quint says that the K18 hair mask is a great treatment if speed is a priority. As she notes above, it works like a conditioner and replaces it in your wash routine. After you towel-dry your hair, apply it, wait four minutes (the brand’s promised time frame for results), then continue with your styling routine.

Quint reported that the results were instant but required a lot of product to avoid tangled hair. (She used five pumps to the brand’s suggested three). Additional caveats: she couldn’t use it with her purple conditioner and the smoothness and shine were less pronounced than when she used the Olaplex treatment.

“There’s no denying that this stuff works,” Quint says. “The effects of this bonding treatment are amazing: Softer, smoother hair that just feels healthier.”

She appreciates that a little goes a long way, reporting that she sees lasting results, even though she only uses the treatment a couple of times a month. Unlike K18, which replaces your conditioner, Olaplex’s No. 3 Hair Repair Perfector is applied pre- shampoo and conditioner. Quint says the results are as close as she can get to a keratin treatment—and at $30/bottle, it’s 90 percent less expensive.

Quint says that both the K18 Molecular Repair Mask and Olaplex No. 3 Hair Repair Perfector produce similar results: smooth, shiny, healthy hair. She personally prefers the latter for more pronounced, longer-lasting results and price ($30 for 100 ML vs. $75 for 50 ML). However, Quint appreciated that the process for using the K18 mask was quicker and held to its promise of results in four minutes. Ultimately, the best product is the one that easily integrates into your lifestyle. Frequently on the go? K18 might be a better choice for you. Have more time to luxuriate? Olaplex is your pick.