The Mucor miehei lipase, Lipozyme, has been found to be vulnerable to temperature-induced deactivation. This thermodeactivation is further aggravated by exposure to butanol. The deleterious effect of the latter on the lipase is a function of its concentration in the aqueous phase containing the enzyme. At 50 degrees C, it was found that a crucial butanol concentration (0.81 M) exists, up to which there is only a slight decrease (25%) in lipase activity. Beyond this, the fall in activity is rapid, with total deactivation at 0.99 M concentration. The deactivation kinetics of the lipase are modeled using a series-type mechanism.