CuInSe2 (CIS) powders were synthesized using CuSe, Cu2Se, and In2Se3 as the raw materials. The formation mechanisms and reaction kinetics from CuSe/In2Se3 and Cu2Se/In2Se3 powders in a selenium atmosphere were investigated. It was observed that the formation temperature of alpha-CIS powders synthesized using Cu2Se/In2Se3 as the raw materials was higher than that using CuSe/In2Se3. Both reactions for Cu2Se/In2Se3 and CuSe/In2Se3 mixtures follow one-dimensional diffusion-controlled reactions with apparent activation energies of 124.3 and 73.2 kJ/mol, respectively. For both mixtures the indium-rich beta-CIS phase resulting from Cu+ ion diffusion toward the In2Se3 phase was observed. The particle size and morphology of the newly formed CIS was similar to In2Se3, which indicated that the CIS formation kinetics may be dominated by the diffusion of Cu+ ions. The Cu-Se liquid phase resulting from the peritectic decomposition of CuSe2 and CuSe at a relatively low temperature may promote Cu+ diffusion into the In2Se3 surface, assisting CIS formation.