The catalytic mechanisms of two esterase-like catalytic antibodies (Abs) have been determined, based on kinetic data and on structures of the complexes with transition-state analogs and with a stable substrate analog of the reactions they catalyze. Both Abs stabilize the oxyanion intermediate close to the transition state in eater hydrolysis. The different geometries of the hydrogen bonds that participate in this stabilization account for most of the difference between the efficiencies of these two Abs.