The keto-enol isomerization of neutral and ionized acetaldehyde, both isolated and catalyzed by solvent molecules, has been studied using the B3LYP and MP2 levels of theory. Single-point calculations at the CCSD(T) level have also been performed. The results show that ionization favors the enolization of acetaldehyde, both thermodynamically and kinetically. Solvent molecules produce an important catalytic effect, especially for the methanol-solvated system, for which the transition state of the isomerization lies below the ground-state asymptote. © 2001 Elsevier Science B.V.