The dependence of initial velocity upon substrate concentration has been examined in the carboxypeptidase A catalyzed hydrolysis of the following hippuric acid esters (at pH7.5, 25°, ionic strength O.5): C6H5CONHCH2CO2CHRCO2H: R=CH3; CH2CH3;(CH2)2CH3; (CH2)3CH3; (CH2)5CH3; CH(CH3)2; CH2CH(CH3)2; C6H5; CH2C6H5. All of these esters display marked substrate inhibition of their enzymic hydrolyses. With the exception of R=CH3, the velocity-substrate concentration profiles for each of these esters can be rationalized by the formation of an E.S2complex which, independent of the alcohol moiety of the ester, reacts approximately 25 times more slowly than the E.S complex. For most of these esters, the formation of E.S2approximates ordered binding of the substrate molecules at the catalytic and inhibitory sites. While binding at the catalytic site is markedly dependent on the nature of the R group, binding of a second substrate molecule to E.S is not significantly affected by the nature of the R side chain. For R=C6H5, theDester is neither a substrate nor a competitive inhibitor of the hydrolysis of theL-ester but can replace theL-ester at the binding site which is responsible for substrate inhibition. The kinetic analysis suggests that this behavior ofDandL-enantiomers is also typical of the other esters examined (except possibly R=CH3). For R=CH3only, substrate activation also seems to occur prior to the onset of substrate inhibition at higher substrate concentrations.