The relationship between water activity (a(w)) and concentration of NaCl, sucrose and various simple and commercial salt, sugar and fat replacers; was determined. The model originally developed by Favetto and Chirife (1985) and extended by Roa and Tapia (1998) was successfully used to model the relationship between a(w) and molality of the binary and multi-component aqueous solutions. These models were then reparameterized to enable the a(w) to be modeled as function of the water phase concentrations of binary and multi-component solutions. This reparameterization enables the modeling of the a(w) depressing effects of compounds for which sufficient data required to calculate the molecular weight cannot be obtained. Both models were successfully used to predict the a(w) of some food products including meat and fish products and mayonnaises with average deviations of 0.0052 from the measured a(w) and bias and accuracy factors of 0.995 and 1.0057, respectively. (C) 2009 Elsevier Ltd. All rights reserved.