Synopsis The aim of this investigation was to find a measure for the phase separation by substances capable of rapidly demulsifying emulsions, which is necessary for the correct determination of the pH in cosmetic W/O emulsions. Results are reported for pH determination in the separated water phase of W/O emulsions containing metal soaps, micropigments and thermally unstable active agents. Of the substances investigated - solvents of different polarity and electrolytes - the best were mixtures of the polar butanol, which has a surfactant-like molecular structure and limited oil and water solubility, and the low polar trichloromethane which in particular has strong fat- and wax-dissolving properties. The combination achieved within 10 to 20 min at room temperature complete phase separations. Stable preparations, based on a polymer emulsifier (cetyl dimethicone copolyol), were demulsified as well as metal soap stabilized creams which characteristically contain a high amount of stabilizing waxes. The so-called solvent phase separation method was confirmed by using model preparations of known pH. During the investigation of model emulsions matching those used in practice, it was found, for example, that magnesium and aluminium stearate as well as various types of micronized zinc oxide which are dissolved or dispersed in the oil phase can shift the pH of the aqueous phase during manufacture from an initial acidic value through neutral to pH 8-10. Time-related and temperature-related changes in pH in formulations containing thermally unstable additives such as dihydroxyacetone or urea can be easily monitored.