Double proton transfers in the prototropic tautomerism of formamide dimer and monohydrated formamide in the gas phase and in solution have been studied as prototypes of multiple proton transfer. The potential energy surface (PES) for the double proton transfer was studied using ab initio quantum mechanical methods, and the solvent effect on the PES was included using the self-consistent reaction field model. In the gas phase, the transition state for the double proton transfer in formamide dimer has C-s symmetry, when the Hartree-Fock (HF) level of theory is used. When the MP2 and B3LYP levels of theory are used to consider electron correlation, the transition state has C-2h symmetry. The double proton transfer occurs concertedly and synchronously. The H bonds in homodimers are stronger than in monohydrated complexes, and the H bonds with formamidic acid are stronger than with formamide. The changes in the H-bond strengths and distances were also calculated as the dielectric constant was increased. The barrier height depends very much on the electron correlation, and the reaction energies of the tautomerization are very sensitive to the size of basis sets. The potential energy barrier for the tautomerization is lowered about 30 kcal mol(-1) in the gas phase by forming hydrogen-bonded dimer. The dimer-assisted tautomerization is kinetically more favorable, but thermodynamically less favorable, than the water-assisted. The tautomerization energies and the potential energy barriers are increased as the dielectric constant is increased both for the water-assisted and for the dimer-assisted reactions, which imply that the tautomerization of formamide becomes less favorable in a polar solvent.