Thin films of compound CuInSe2 have been developed onto glass substrates by in situ thermal annealing of the stack of successively evaporated elemental layers in vacuum.; The atomic compositions and the optical properties of the films have been determined by proton-induced X-ray emission (PIXE) method and spectrophotometry in the photon wavelength range of 300-2500 nm, respectively. The typical optical absorption characteristic of the films has been critically analysed. The absorption coefficients vary from 10(3) to 10(5) cm(-1) the measured wavelength range. The films have more than one type of fundamental electronic transitions. Direct allowed and direct forbidden transitions vary between 0.947 to 0.989 eV and 1.099 to 1.204 eV, respectively, depending on the composition of the films. The former transition varies inversely with the Cu/In ratio while the latter shows no such dependence. Valence band splittings due to spin-orbit coupling converge towards the single-crystal value for the near-stoichiometric (NS) and Cu-rich films.