Optical properties of CuInSe2 films, deposited by the three-source evaporation technique at a substrate temperature of 670 K, were studied critically to determine the grain boundary scattering effects from the tailing of the absorption edge. The experimental data were analysed in the light of the grain boundary trapping model in polycrystalline films. The crystallites were found to be partially depleted of carriers. The barrier height along with the carrier concentration in the films were determined from optical reflectance measurements. The effect of the variation of the Cu/In ratio (0.4-1.18) on the grain boundary scattering parameters has been critically discussed. The density of trap states in the intercrystalline region of the films was found to increase with increasing Cu/In ratio. The built-in electric field in the crystallites was evaluated for different sizes of the crystallites. The effect of illumination on the grain boundary barrier height was studied and this indicated that the grain boundary charges are depleted by subbandgap photons resulting in the reduction of the barrier height with increase of illumination level. © 1994.