Kiehl's Ultra Pure High-Potency 9.8% Glycolic Acid Serum Review

The most telling thing on this bottle is the decimal. Kiehl's could have rounded to ten; it wrote 9.8, and the reason is that ten percent is the ceiling the cosmetic industry's own safety panel set decades ago for alpha-hydroxy acids sold for home use, at a pH of 3.5 or above. A serum labelled 9.8 is a serum sitting a hair under that line on purpose, and it tells you two things before you open it: that the brand is being precise about a number most competitors fudge, and that you are about to put nearly the maximum permitted at-home glycolic acid on your face with, as the seven-ingredient list confirms, almost nothing to cushion it.

The list is water, glycolic acid, glycerin, denatured alcohol, sodium hydroxide, sodium hyaluronate, hydroxyethylcellulose. That is a working acid and its minimum support: glycerin to hold water while the acid loosens dead cells, alcohol fourth to thin the formula and speed penetration, sodium hydroxide to bring the acid partway up from its raw pH toward the mid-threes where it exfoliates without burning, a trace of hyaluronate for a softer finish, a thickener for body. No fragrance, no botanicals, no soothing agents, no second acid. In its minimalist line Kiehl's built a hyaluronic serum that was gentle because it was empty; this one is strong for the same reason.

On skin the strength is immediate and honest. The watery liquid spreads and flashes off within a minute, and as it goes there is a sting, a sharp tingle that builds for twenty or thirty seconds and settles inside a couple of minutes, followed by a faint warmth. That is ten percent glycolic acid and alcohol meeting clean skin, and it is the sensation of the product working rather than a warning, provided it fades. The next morning the skin is smoother to the touch; within a week, which is where Kiehl's one-week grading and its ninety-eight percent softer-skin figure land, it looks brighter and more even, pores read tighter, and the small congestion on the nose and chin begins to loosen. Over a month or two of use two or three nights a week, post-blemish marks fade and texture refines in the way glycolic acid has been documented to do since the 1990s. Four hundred reviewers describe the same arc, and the ones who stopped early are the ones the formula was not built for.

There is a useful way to use it that the brand mentions and most buyers skip: zone treatment. Two drops on the nose, chin and forehead, where combination skin actually has the congestion and texture, and nothing on the cheeks, gets most of the benefit with a fraction of the exposure, and it is how a bottle this strong earns a place in a routine that also has room for a retinoid on other nights. Used that way, a few drops a couple of evenings a week, the thirty millilitres lasts half a year, which is the only sense in which the price becomes reasonable.

Who it was built for is oily, combination and resilient normal skin. Alcohol fourth on the list makes the serum weightless and quick, and it also makes it drying, so dry skin will feel tight by the second use and sensitive, rosacea-prone or barrier-compromised skin should not go near it; there are gentler acids, and gentler glycolic serums, for those faces. The sun-sensitivity that any alpha-hydroxy acid brings is real and lasts for days after each use, which makes daily sunscreen a condition of buying the bottle rather than advice. Retinoids, other acids and vitamin C belong on different nights. Pregnancy is one of the few situations where glycolic acid is usually preferred to a retinoid, since so little is absorbed, and this serum's short list is an advantage there, provided the skin is not already reactive.

The real argument is price, and it is a losing one for the formula. Forty dollars for thirty millilitres of seven ingredients that cost pennies to combine is luxury pricing on a value-brand recipe, and the acid does not know who bottled it or what the dropper is made of. Ten percent glycolic serums with better buffering and no alcohol sell for a fraction of the amount, and a reader who simply wants the exfoliation should buy one of them. What Kiehl's charges for is the apothecary name, a glass dropper, L'Oréal's expert grading and the unusual honesty of the label, and the economy of use, two or three drops a couple of nights a week for four to six months, at least keeps the cost per application modest.

The recommendation, then, is narrow and clear. Oily or combination skin that wants a strong, clean, fragrance-free glycolic acid with a number it can trust, and that already shops the brand, will get exactly what the decimal promises. Everyone else should note, without rancour, that the same strength is available for less, that the same gentleness is not available in this bottle at all, and that 9.8 is a fine number to admire and a hard one to wear.