A combination kinetic model has been used to explain the dual characteristics of kinetic behavior of kid pregastric lipase (PGL) catalyzed hydrolysis of 4-nitrophenyl butyrate and tributyrin in aqueous solution in the presence of surfactants. At low concentrations of surfactant, the activity followed the behavior of an esterase, with a strong preference for catalyzing the hydrolysis of the water-soluble form of the substrate, and the presence of a surfactant served as a solublizer which dissolved and increased the concentration of this solubilized form. However, at high concentrations of surfactant, the PGL acted mainly as a lipase which catalyzed the hydrolysis of substrate aggregates. We have concluded that kid PGL acts as both a lipase and an esterase, and which of these factors is dominant is decided by the reaction conditions. By using this model, it has also been possible to explain the confusion which exists in the literature over the apparent effects of bile salts on the activity of preduodenal lipases. When a bile salt was present, it did not have a molecular interaction with the PGL, but served only as a surfactant in the catalyzed hydrolysis reaction which followed dissolution/encapsulation of the substrate in solutions of increasing bile-salt concentration.