Synopsis A number of alpha-hydroxy acids, including glycolic, lactic (l(+) and d(-) forms) citric, hydroxybutyric and malic acids, were evaluated for their effects on various skin properties at concentrations of 0.5-1.15 m. The acids were evaluated for their stinging potential on sensitive skin, the ability to increase skin cell renewal, and their ability to improve moisture content and reduce lines and wrinkles over a six-week period. Results indicated that lactic l(+) and glycolic were the most effective acids when tested at equimolar concentrations. Both materials were less irritating than the d(-) lactic acid form and the other acids tested. Even though with long-term use both d(-) and l(+) lactic acid and glycolic acid produced comparative improvements in skin hydration and lines and wrinkle improvement, the l(+) form of lactic acid did so with fewer consumer complaints than any of the other acids tested. When the concentration of lactic and glycolic acids was increased to more than 1.5 m a greater difference in irritation potential was observed with respect to stinging, with the l(+) form of lactic acid being less irritating than glycolic acid and the d(-) form. Our results suggest that l(+) and d(-) lactic acid and glycolic acid are the two most effective materials for developing alpha-hydroxy acid products. Lactic acid has a slight technical advantage over glycolic acid. Both l(+) and the d(-) forms are better moisturizers when fully or partially neutralized. Moreover the l(+) form is also less irritating than d(-) and glycolic acid. Other acids such as citric, malic, and hydroxybutyric acids appear to be less effective materials but may be useful additions to lactic or glycolic acid mixes.