We have investigated the reflectivity of GaAs thin layers in the excitonic absorption region, observing detailed interference structures which reveal the polariton dispersion and the quantization of the exciton centre-of-mass motion. To analyse the experimental spectra, we have calculated the optical properties of thin films using a real-space density-matrix approach adapted to the case of degenerate valence bands. The oscillatory behaviour of the reflectance is well reproduced ; a surface layer where the polarization decreases to zero is computed and its thickness is found to be energy dependent. The temperature dependence shows a broadening of the fine structure due to the phonon interaction and a decoupling of excitons and photons at high temperature