The deactivation kinetics of yeast alcohol dehydrogenase (YADH) in both aerosol OT/isooctane reverse micelles and aqueous buffer were studied. The YADH entrapped in reverse micelles could retain activity for above 24 hr although it was less stable than dissolved in aqueous buffer. Both the activity-time curves for the YADH in reverse micelles and in aqueous buffer exhibited a rather rapid exponential decay within the early 2 hr, followed by a slower exponential decay during the remaining period. A series-type enzyme deactivation model involving two first-order steps and one active intermediate was used to describe the deactivation behavior of YADH. The kinetic parameters of the deactivation rate equations were obtained by optimization method. In aqueous buffer, the deactivation rate of YADH exhibited a maximum around a Tris concentration of 0.1 mol m(-3). The deactivation rate of YADH in reverse micelles was strongly dependent on Tris concentration and the molar ratio of water to surfactant (omega(o)). The residual activity percentage of the active intermediate increased with the increase of omega(o) and Tris concentration, while both the rate constants for the first and second first-order deactivation steps decreased with the increase of Tris concentration.