To determine the capability of formic acid as a hydrogenating agent in supercritical water, we conducted hydrogenation of acetone using formic acid. It was found that the formic acid enabled hydrogenation of acetone to produce 2-propanol. We varied the concentration of formic acid available for hydrogenation by changing the retention time in the formic-acid preheater, prior to mixing with acetone. The highest yield of 2-propanol occurred when the concentration of formic acid was a maximum. The yield decreased with thermal decomposition of the formic acid, as the retention time in the preheater was increased. However, an unexpected and rapid increase in the 2-propanol yield was observed when the retention time in the preheater was further increased. We theorize that the hydrogenating agent responsible for this phenomenon is closely related with the presence of carbon monoxide although further work is required to elucidate the mechanism.