The dependence of the catalytic activities of α-chymotrypsin and laccase on the concentration of organic cosolvents (alcohols, glycols and formamides) in mixed aqueous media has a pronounced threshold character: it does not change up to a critical concentration of the non-aqueous cosolvents added, yet further increase of the latter (by only a small percentage, by vol.) leads to an abrupt decrease in enzyme activity. Fluorescence studies indicate that the inactivation results from reversible conformational changes (denaturation) of the enzymes. There is a linear correlation between the critical concentration of residual water (at which the enzyme inactivation occurs in a threshold manner) and the hydrophobicity of the organic cosolvents added. A quantitative criterion is suggested for the selection of organic cosolvents to be used for enzymatic reactions in homogeneous water/organic solvent media.