When studying biocatalysts in organic media, it is useful to be able to convert between water concentration and thermodynamic water activity (a(w)). We have obtained relationships for this purpose based on published vapor-liquid equilibrium data for a range of commonly used polar solvents. Their use is illustrated by the reanalysis of literature data for the activity of an immobilized laccase. Kinetic constants measured as a function of water concentration can thus be related to a(w). We show that the critical water content for significant V-m values is characterized by a similar a(w) (about 0.4) in five different alcohols; however; the critical a(w) becomes progressively higher in other water-miscible solvents in the order : dioxane, acetonitrile, tetrahydrofuran, acetone (critical a(w) about 0.8). This indicates that such polar solvents probably have important direct effects on the enzyme.