This article describes the investigation of the molecular mechanism of the catalytic antibodies elicited against a phosphonate transition state analog. We have investigated the biochemical properties within a panel of six hydrolytic catalytic antibodies. The transition-state analysis (kcatkuncat versus Km/ Ki) displayed a linear relationship with five antibodies, 6D9, 8D11, 4D5, 3G6, and 9C10, which have homologous primary amino acid sequences, while 7C8 deviates from the linear relationship. These results suggest that the five antibodies catalyze the hydrolysis by the same mechanism of transition-state stabilization and that factors other than transition state-stabilization are involved for antibody 7C8. The structures of complexes of 6D9 Fab and 7C8 Fab with the transition-state analog (TSA) have been solved. Antibodies 6D9 and 7C8 bind the phosphonate of TSA by forming one hydrogen bond with His L27d and TyrH95, respectively. The kinetic, structural, and thermodynamic analyses of 6D9 indicate that this antibody provides an efficient catalyst by the transition state stabilization through the hydrogen bond with HisL27d and the destabilization of the substrate. The most probable catalytic mechanism of the 7C8-catalyzed hydrolysis of an ester is the nucleophilic catalysis by the deprotonated TyrH95. Our results demonstrate that substantial diversity may be present among the antibodies raised against a single TSA and which catalyze the same reaction.