Kinetics of lipase-catalyzed hydrolysis of esters were modeled using reactant activities for aqueous-organic, biphasic systems. By using thermodynamic activities of the substrates in ordinary rate equations, the kinetic parameters were corrected for the contribution of substrate-solvent interactions and a uniform quantification of the substrates for lipase attached to the interface can be achieved. The kinetic parameters, on the basis of their thermodynamic activities, should be constant in different systems, provided that the solvents do not interfere with the binding of the substrates to the enzyme nor affect the catalytic mechanism. Experimental and computational methods on how to obtain the thermodynamic activities of the substrates are presented. Initial rates were determined for Pseudomonas cepacia lipase (PcL)-catalyzed hydrolysis of decyl chloroacetate in dynamic emulsions with various solvents. The thermodynamic equilibrium and corrected kinetic constants for this reaction appeared to be similar in various systems. The kinetics of Pet in an isooctane-aqueous biphasic system could be adequately described with the rate equation for a ping-pong mechanism. The observed inhibitory effect of decanol appeared to be a consequence of this mechanism, allowing the backreaction of the decanol with the chloroacetyl-enzyme complex. The kinetic performance of Pet in systems with toluene, dibutyl ether, and methyl isobutyl ketone could be less well described. The possible causes for this and for the remaining differences in corrected kinetic parameters are discussed.