Cu(In,Ga)Se 2 films were successfully synthesized via employing H 2 SeO 3 as a new Se source. Direct heating in a reducing atmosphere at 350°C without using Se vapor yielded monophasic Cu(In,Ga)Se 2 . H 2 SeO 3 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)Se 2 were investigated, and the related reaction mechanism was proposed in the study. H 2 SeO 3 is first decomposed to yield selenium species at elevated temperatures. Subsequently, In 2 Se 3 is formed from selenium and indium species. Finally, In 2 Se 3 reacts with other species to yield Cu(In,Ga)Se 2 . In the heating process, decreasing the heating rates resulted in a significant increase in the grain size and the compactness of the prepared films.