[en] Photoacoustic spectroscopy (PAS) has proved to be an effective technique for evaluating inherent defects in a wide range of materials. This paper demonstrate the use of this technique to evaluate the optical properties of CuIn/sub 0.75/Ga/sub 0.25/Se/sub 2/thin films. Films were grown by flash evaporation technique onto glass substrates the temperature of which was varied from room temperature to 200 deg. C. The as-grown samples were first annealed in selenium atmosphere and then implanted with hydrogen species. A high resolution near-infrared photoacoustic spectrometer of the gas-microphone type is used for room temperature analysis (in the sub gap region of the spectrum) of non-radiative defect levels in the as-grown, annealed and hydrogen implanted thin films. The absorption coefficient has been derived from the photoacoustic spectra to determine the gap energy and establish the activation energies for several defect-related energy levels. The changes observed in the photoacoustic spectrum following annealing and ion implantation are directly correlated with the compositional and structural properties of the sample. (author)