[en] Carbon-13 kinetic isotope effects were determined for the decarboxylation of pyruvate by thiamin and for the decarboxylation of 2-(1-carboxy-1-hydroxyethyl)-3,4-dimethylthiazolium chloride (a covalent pyruvate-thiamin adduct) in water and in aqueous ethanol. The kinetic isotope effect for the decarboxylation step in the second system increased from 1.051 in H₂O to approximately 1.058 in 50% v/v ethanol. The isotope effect in the thiamin catalyzed model reaction increased from the 0.992 inverse effect observed in H₂O to 1.007 in 50% aqueous ethanol. The rate of decomposition of the covalent thiamin-pyruvate adduct to reactants relative to its rate of decarboxylation is greater in ethanol than in water. Further, the results suggest that the changes of the vibrational force constants in going from the ground state to the transition state are greater in aqueous ethanol than in water