Cu(In,Ga)Se2 films were successfully synthesized via employing H2 SeO3 as a new Se source. Direct heating in a reducing atmosphere at 350°C without using Se vapor yielded monophasic Cu(In,Ga)Se2. H2SeO3 was completely dissolved in the starting solution, leading to an improvement in homogeneous mixing and diffusion of selenium atoms with other constituent species in the precursor pastes. The formation processes of Cu(In,Ga)Se2 were investigated, and the related reaction mechanism was proposed in the study. H2SeO3 is first decomposed to yield selenium species at elevated temperatures. Subsequently, In2Se3 is formed from selenium and indium species. Finally, In2Se3 reacts with other species to yield Cu(In,Ga)Se2. In the heating process, decreasing the heating rates resulted in a significant increase in the grain size and the compactness of the prepared films.