CuInSe2 (CIS) thin films with a range of Cu/In ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of T-s = 450-500 degrees C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of T-A = 350 degrees C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In2O3 phase was found both in Cu-rich and In-rich films annealed at T-A greater than or equal to 3.50 degrees C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the Cu-In-Se-O-N system showed In(2)O(3)2 to be the most stable phase in good agreement with the experimental results.