Ab initio calculations at various levels (up to QCISD(T) and MP4/6-311+G(d,p)//MP2/6-31G(d)) and Monte Carlo simulations were used to investigate protonated formamide and formimidic acid, which are presumed intermediates in the acid-catalyzed proton exchange in formamide. Both in the gas phase and in aqueous solution, protonation at the oxygen leads to a substantially more stable intermediate than protonation at the nitrogen. Of the two conformers of the O-protonated intermediate, one is preferred in the gas phase, but they become nearly equal in solution. Among the four conformers of formimidic acid, one is preferred in the gas phase, but in solution another conformer is preferred, with two others within 1 kcal/mol. Thus, solvation plays an important role in determining relative stabilities of these intermediates.