Background/Aims: The effects of a 10% alpha-hydroxy acid (AHA) oil/water (O/W) emulsion on the pH of human skin surface (pH(ss)) and stratum corneum (SC; pH(ss)) were evaluated in vivo. Methods: The AHA O/W emulsion was applied to an area on the volar forearm of male volunteers (n = 12), and then wiped off after 10 min. Prior to application and over the following 3 h, the pH(ss) was measured. We used glass electrode measurements and time domain dual lifetime referencing (tdDLR) with luminescent sensor foils. In another experiment (n = 5), the impact of the AHA O/W emulsion on the pH(sc) gradient was measured by tape stripping of the SC of the volar forearm after application of the AHA O/W emulsion. Results: Compared with pH(ss) values prior to treatment [5.2 +/- 1.7 (tdDLR) or 4.8 +/- 0.5 (electrode)], the pH(ss) was significantly reduced 10 min after application [4.0 +/- 0.3 (tdDLR) or 4.1 +/- 0.1 (electrode)] and the pH(ss) remained significantly reduced over the measurement period of 3 h [after 3 h: 4.4 +/- 0.2 (tdDLR) or 4.5 +/- 0.3 (electrode)]. The AHA O/W emulsion significantly reduced the pH(sc) even down to deep layers of the SC. Conclusion: After a 10-min application time, the 10% AHA O/W emulsion reduces the pH(ss) (for at least 3 h) and pH(sc) in deep layers of the SC. copyright (C) 2011 S. Karger AG, Basel