The photocatalytic behavior of pure and supported Mo2C was investigated in the vapor-phase decomposition of formic acid. IR studies showed that illumination promoted the dissociation of molecularly adsorbed HCOOH and the decomposition of formate species formed on Mo2C/TiO2 sample. Mo2C prepared on TiO2 enhanced the extent of the photocatalytic decomposition exhibited by TiO2. Both the dehydrogenation and dehydration reactions occurred. Interestingly, Mo2C had photocatalytic properties even in unsupported state and also on an inert SiO2 surface. Its photoactivity was dramatically increased by the presence of potassium prepared by the carburization of K2MoO4, when the dehydrogenation came into prominence. Pure and K-doped Mo2C showed an appreciable photocatalytic effect in the visible light, which was attributed to their lower bandgap.