Both thermodynamic and kinetic parameters are important in food processing and stability. Thermodynamic parameters are represented by water activity and generally applied to high and medium moisture content materials, where molecules can freely diffuse, without restrictions. In contrast, kinetic properties mainly apply to intermediate and low moisture content foods, where diffusional mobility of reactants is more or less restricted and where water acts as a plasticizer of amorphous compounds. Kinetic properties can be represented by viscosity and related to the glass transition temperature (T-g) by the Williams-Landel-Ferry equation. Thus, in a food system passing From high to intermediate and low moisture content and/or from high to low temperature, the prevailing control mechanism will gradually change from thermodynamic to kinetic. The relationship between the water activity, temperature and viscosity is presented for some model solutions.