The water activity, pH and other compositional parameters such as the moisture, fat and NaCl contents of various food emulsions were determined: butters, margarine (W/0) and mayonnaises (O/W). Water activity was determined by a gravimetric method (PEC procedure) and an instrumental method (dew-point hygrometer). The mean a(w) values obtained for the samples assayed were 0.904 +/- 0.050 (butters), 0.914 +/- 0.028 (margarines) and 0.947 +/- 0.013 (mayonnaises). The a(w) of the butter and margarine samples could be predicted with an error less than 0.02 units from the NaCl content of their aqueous phase (m) by using the following simple linear regression equation: a(w) = 0.954 - 0.036m. This equation could not be used to predict the a(w) of the mayonnaises, which, however, were obtained by using the Chen equation for mixtures of solutes.