Here we present a kinetic study on the steroselectivity and regioselectivity of 23 purified lipases of animal and microbial origin. This work, concerning a general problem of the mechanism of lipase-substrate molecular recognition, was performed using pure dicaprin isomers: 1, 2-sn-dicaprin, 2,3-sn-dicaprin, and 1,3-sn-dicaprin spread as monomolecular films at the air-water interface. The first two isomers are optically active antipodes (enantiomers), forming stable films up to 40 mN m-', whde the last is a prochiral compound, with a surface pressure of collapse of 32 mN m-'. To our knowledge, this is the first report on the use of three diglyceride isomers as lipase substrates under identical and controlled physicochemical conditions. The lipases tested display a typical behaviour, characteristic of each enzyme, which allowed us to classlfy the lipases in groups according to (1) the profiles of enzyme velocity as a function of surface pressure, (2) their preferences for a given diglyceride isomer, quantified using new parameters termed steroselectivity index (S.I.), vicinity index (V.I.), and surface pressure threshold (S.P.T.). The general observation, true for all the enzymes tested, is that the three substrates are well differentiated, and the differentiation is more pronounced at high interfacial energy (low surface pressure). This observation supports our hypothesis that lipase conformational changes, resulting from the enzymesurface interaction, affect the enzymes' speciiicities. Generally speaking, the stereopreference for either sn-1 or sn-3 position on glycerides is maintained both in the case of di-and tri-glycerides.