A covalent imminium adduct, formed by condensation of aminomalonate with acetone, undergoes decarboxylation (k = 0.03 s-1 at 25 degrees C) in water 30 000 times more rapidly than does aminomalonate in the absence of acetone. A Brønsted plot of the observed rates of decarboxylation of these and other ionized carboxylic acids, as a function of the pKC-H values of the carbon acids generated by their decarboxylation, exhibits a betalg value of 0.7, indicating that the structures of the transition states for decarboxylation of the carboxylate forms of these acids approaches the structures of the carbanions generated by their decarboxylation. On the basis of an estimated pKC-H value for benzene in water ( approximately 43), extrapolation of that Brønsted plot leads to the prediction that benzoate decarboxylation should proceed at detectable rates in water at temperatures below the critical point. That prediction was confirmed experimentally. Using this same relationship, and extrapolating to the observed rate constant for enzymatic decarboxylation of orotidine 5'-monophosphate, we estimate that the "effective" pKa value of the 6-CH group of uridine 5'-monophosphate, the product of decarboxylation, is 9.5 at the active site of yeast OMP decarboxylase.