The hydrolysis of alkyl enol ethers to their corresponding carbonyl compounds proceeds by acid-catalyzed, rate-determining protonation on the beta-carbon to form an oxocarbonium ion intermediate (Kresge, A. J.; Chang, Y. J. Chem. Soc. B 1967, 53). Antibody 14D9 (anti-1) catalyzes the hydrolysis of enol ethers 4 and 5 with very high enantioselectivity of protonation (Reymond, J.-L.; Janda, K. D.; Lerner, R. A, J, Am, Chem. Soc, 1992, 114, 2257). Catalysis involves participation of an antibody side chain as a general acid, as well as pyramidalization of the enol ether’s beta-carbon by hydrophobic contacts between its substituents and the antibody (Reymond, J.-L.; Jahangiri, G. K.; Stoudt, C.; Lerner, R. A, J. Am. Chem. Soc. 1993, 115, 3909). The present study addresses the question of the origin of the enantioselectivity of this catalyst. First, enantioselectivity and substrate tolerance, which are most remarkable in antibody 14D9, are shown to be recurrent features for anti-1 or anti-2 antibodies. Four antibodies were studied, and all enantioselectively deliver a proton on the re face of enol ethers to produce (S)-configured carbonyl products, while stereoselectively binding to analogs of the (S,S)-hapten 1. The orientation of the enol ether at the transition state relative to the hapten is then established by comparing the effect of alkyl substitutions at the beta-carbon on antibody catalysis with the effect of equivalent substitutions on antibody binding to hapten analogs. For antibody 14D9 (anti-1), the results show that the alkyl substituent of the enol ether’s beta-carbon binds to the N-methyl site of the hapten at the transition state. Substitution of ethyl for methyl at that position results in a 20-fold drop in transition state binding and a 3-7-fold drop in affinity for inhibitors. The orientation is such that the cyclic substrates do not fit in the site complementary to the piperidine ring of the hapten at the transition state. The antibody-catalyzed hydrolysis of the cyclopentanone enol ether 6, which produces exclusively (S)-7, is 40 times more efficient than for the cyclohexanone enol ether 10. By contrast, no binding selectivity is found for the individual enantiomers of the corresponding ketone products 7 and 18, which are neutral transition state analogs for re- or si-selective protonation of 6 or 10. The enantioselectivity of 14D9 appears only for the transition state, which suggests that it contains a dynamic component, probably the strict geometrical constraint that the enol ether be aligned with the antibody residue acting as a general acid catalyst during proton transfer. The enantioselectivity of antibody 14D9 thus results from an unexpected combination of binding and catalysis. This study establishes the relationship between hapten and transition states in unprecedented details. The observation that the discriminating power of an antibody for enantiomeric transition states can far exceed simple binding discrimination for ground state molecules suggests a promising future for catalytic antibodies as enantioselective catalysts.