Enzyme catalysis in aqueous-organic cosolvent mixtures has wide applications. However, inadequate attention has been paid to the issue of stability of enzymes in such media. The results with polyphenol oxidase, peroxidase, acid phosphatase, and trypsin show that solvents with polarity indexes of 5.8 and above are "good" solvents. These solvents when used as cosolvents in aqueous-organic solvent media do not denature the enzymes irreversibly. Enzyme(s) exposed to these solvents retain most of their activity even after 48 h of exposure, whereas solvents with polarity indexes of <5.1 denature the enzyme completely within 0-4 h in most of the cases studied. It appears that at higher concentrations (50% and above) cosolvents effectively compete with the water layer around the enzyme. Fluorescence spectroscopy shows that, although the presence of all the organic cosolvents cause conformational changes in the enzyme molecule at a concentration of 50% (v/v), these changes were completely reversible (when the concentration of organic solvent is diluted with aqueous buffer) in case of solvents having polarity indexes of 5.8 and above. In cases of the solvents having polarity indexes of 5.0 and below, the exposure at 50% concentration changed the conformation of the enzymes irreversibly. Thus, a simple parameter, viz. polarity index, may help in medium engineering of enzyme catalysis in nonaqueous surroundings.