Formic acid catalyzed tautomeric conversion of formamide to formamidic acid has been investigated by use of ab initio and density functional theoretical calculations. In a 1: 1 dimeric complex between formamide and formic acid, the tautomeric conversion occurs via double-hydrogen transfer within an eight-member hydrogen-bonded cyclic network. The results predict that the energy barrier of the catalytic process is reduced by more than a factor of 4 compared to that in the isolated formamide molecule in the gas phase, and the tautomerization in the 1:1 complex is several kcal/mol less endothermic than that of the isolated molecule. The potential energy surface corresponding to this double hydrogen transfer process indicates that a concerted transfer of both the hydrogen atoms along the hydrogen bond directions is energetically favorable, and no minimum for an ionic intermediate, which may arise for stepwise transfer, was predicted. The unique configuration of the transition state has been identified by starting the reaction from both the tautomeric forms, and the transition state was subjected to IRC calculation.