The kinetic behavior of yeast alcohol dehydrogenase (YADH) in sodium di-2-ethylhexylsulfosuccinate (AOT) isooctane reverse micelles has been studied using methyl ethyl ketone (MEK), NADH and Tris as the substrate, coenzyme and buffer, respectively. The solubility diagrams of aqueous buffer in the isooctane solution of AOT were established as a function of temperature and molar ratio of water to surfactant (wo) at various Tris and AOT concentrations. The dependence of enzyme activity on enzyme concentration, pH, wo and Tris concentration was determined. The optimal w o was at 10-15, increasing slightly with an increase in Tris concentration. The YADH entrapped in reverse micelles exhibited minimum activity at a Tris concentration of 0.1 rnol dm-3, while in aqueous buffer enzyme activity was not significantly affected by Tris concentration. Comparing the rate equation for the reduction of MEK by YADH in reverse micelles with that in aqueous buffer, the association of YADH and NADH could apparently have proceeded with an irreversible reaction before the substrate was bound, when performed in a reverse micellar system. Although the YADH entrapped in reverse micelles was less stable than when dissolved in aqueous buffer, the enzyme retained at least 20% activity after 21 h at 25Β°C and wo = 20. This result was an improvement over previously reported data.