Formic acid (HCOOH) decomposes efficiently to afford H2 and CO2 selectively in the presence of a catalytic amount of a water-soluble rhodium aqua complex, [RhIII(Cp*)(bpy)(H2O)]2+(Cp*=pentamethylcyclopentadienyl, bpy=2,2′-bipyridine) in aqueous solution at 298 K. No CO was produced in this catalytic decomposition of HCOOH. The decomposition rate reached a maximum value at pH 3.8. No deterioration of the catalyst was observed during the catalytic decomposition of HCOOH, and the catalytic activity remained the same for the repeated addition of HCOOH. The rhodium-hydride complex was detected as the catalytic active species that undergoes efficient H/D exchange with water. When the catalytic decomposition of HCOOH was performed in D2O, D2 was produced selectively. Such an efficient H/D exchange and the observation of a deuterium kinetic isotope effect in the catalytic decomposition of DCOOH in H2O provide valuable mechanistic insight into this efficient and selective decomposition process.