The catalytic activities of lipases derived from Pseudomonas sp and pretreated with various organic solvents were investigated. The activity of the solvent-pretreated lipase was greater than that of native lipase in both the esterification reaction in an organic medium and the hydrolysis reaction in an,aqueous medium. With esterification calalysed by pretreated lipase, the product, benzyl octanoate, was detected without time-lag. Conversions at equilibrium state were correlated with the hydrophobicities of the solvents used. In the hydrolysis reaction, most pretreated lipases yielded increased acid production compared with native lipase. A linear correlation was observed between the solvent hydrophobicity and the relative initial reaction rate of the hydrolysis reaction when using pretreated lipases.