To elucidate the details of the esterase activity of human serum albumin, the reaction of N-trans-cinnamoylimidazoles with albumin was investigated kinetically at various pHs at 25 degrees. The reaction consisted of the acylation of albumin (probably the tyrosine-411 residue) by the substrate and the deacylation of cinnamoyl-albumin. The acylation was approximately 10--100-fold faster than the spontaneous hydrolysis of the substrate over the pH range examined. The pH profile for the deacylation rate constant indicated the participation of a group having a pKa of approximately 9.4. The deacylation was subjected to the effect of deuterium oxide. The electron-withdrawing substituent facilitated the deacylation; the Hammett rho value was 1.63. These results suggest that the deacylation proceeded via general base catalysis by this group.