We report a detailed study of the properties of exciton polaritons confined in GaAs thin layers grown by molecular-beam epitaxy. Intermediate-layer thicknesses from 150 to 600 nm between quasi-twodimensional quantum wells and bulk GaAs are considered. Both reflectance and photoluminescence measurements have been performed, and a large number of oscillation fringes and emission peaks have been detected. The spectra obtained are significantly different from those of a quantum well or of bulk GaAs, and can be attributed to quantized levels of the exciton-center-of-mass motion. The thickness dependence of the exciton peaks shows dominant contributions from the heavy-hole exciton dispersion due to its larger oscillator strength and density of states. Polariton effects and interference between different modes have to be taken into account to explain the reAectance behavior. This is done using three types of additional boundary conditions (ABC' s). From the sensitive dependence of the reflectance line shape on different ABC' s, we conclude that only Pekar's ABC gives interference fringes comparable to the experimental data. A temperature-dependence investigation gives clear evidence that the polariton effects manifest themselves in both optically thin and optically thick layers of GaAs only at low ternperatures. At higher temperatures the optical transitions are more excitonlike. The results are discussed by modeling the exciton-broadening parameter as a function of the exciton-phonon interaction.