Using molecular beams, polycrystalline thin CuInS2 (CIS) films of different thicknesses were grown on Si substrates covered with a sputtered Mo-buffer layer. Systematic photoluminescence and photoreflectance measurements were performed to investigate the influence of strain - introduced during growth - on the optical properties. The transition energy of the free A-exciton (FXA) decreases with increasing tensile strain caused by (i) increasing thickness of the Mo buffer layer and (ii) decreasing thickness of the CIS layer. Furthermore, the energetic splittings between FXA, FXB, and FXC increase with increasing tensile strain. When combined with X-ray diffraction data, the oscillator strengths of the excitonic transitions yield information on the strain distribution within the films.