Single phase chalcopyrite CuInSe2(CIS) phase formation at low temperatures (≊260 °C) is demonstrated by Se vapor selenization of evaporated metal precursors at a pressure of ≊0.3 mbar. The low pressure changes the relative kinetics of selenization of Cu and In due to the modification of the availability of Se reacting species. Consequently, the thermodynamically favorable reaction CuSe+In(1)+Se→CIS proceeds to completion at lower temperatures. This is confirmed by the absence of In2Se3at intermediate pressures (0.5–1.0 mbar). At higher pressures (≊7–10 mbar) and Se flux, simultaneous formation of the equilibrium binaries, CuSe and In2Se3at low temperatures, leads to the formation of CIS through a diffusion limited reaction of the binaries at higher (≥400 °C) temperatures. ©1995 American Institute of Physics.