Optothermal infrared spectrometry (OTIS) is a novel way of measuring the water content of stratum corneum non-invasively. This principle has been used in the present study to evaluate the hydrating effect of a one-week treatment of human skin twice a day with either white petrolatum or a cream (o/w emulsion). Forty-two females volunteered for the study, which comprised one control pretreatment week, one treatment week, and one post-treatment week. White petrolatum was greasy and did not produce any hydrating effect at any point in time when the hydration was measured 10 h after application, whereas the cream produced a clear hydration that became statistically significant from day one of treatment and was maintained for at least 2 days after the treatment was stopped. The hydrating effect amounted to an about 80% increase in those volunteers who had initial values below the mean OTIS value of 23.5%. It is concluded that saturation of the stratum corneum with appropriate lipids and emulsifiers as in the cream leads to hydration of the stratum corneum to about 35% water as measured by the OTIS technique.