The rate constants for deprotonation of ethyl acetate by 3-substituted quinuclidines are correlated by β ) 1.09 ( 0.05. The limits of k BH ) 2-5 × 10 9 M -1 s -1 for the encounter-limited reaction of the simple oxygen ester enolate with protonated quinuclidine (pK BH ) 11.5) were combined with k B ) 2.4 × 10 -5 M -1 s -1 for deprotonation of ethyl acetate by quinuclidine, to give pK a K) 25.6 ( 0.5 for ionization of ethyl acetate as a carbon acid in aqueous solution. A rate-equilibrium correlation for proton transfer from methyl and benzylic monocarbonyl compounds to hydroxide ion has been extended by 6 pK units in the thermodynamically unfavorable direction, and it is shown that the absence of curvature of this correlation is inconsistent with a constant Marcus intrinsic barrier for the enolization of simple carbonyl compounds.