The decarboxylation of mandelylthiamin is subject to general base catalysis (beta = 0.26), an outcome that is inconsistent with the expected dissociative transition state in which CO2 forms along with a residual carbanion. The results implicate a previously unrecognized associative route in which addition of water to a carboxylate followed by base-catalyzed proton transfer and C-C cleavage produces bicarbonate directly. Various reports of the presence or absence of base catalysis in decarboxylation reactions are consistent with the associative route's occurrence in cases where nucleophiles would be generated along with CO2 in the usual dissociative route.