An analytical method for quantitative interpretation of GaAs photoluminescence spectra was developed. Because of various transition mechanisms the photoluminescence spectrum of a sample may vary significantly under different measurement conditions. Based on a proposed scheme of transition priorities, spectra taken at various excitation powers were analyzed. Comparing results of undoped GaAs epitaxial layers grown by organometallic chemical vapor deposition under similar conditions but different V/III ratios, an optimum ratio corresponding to a minimum number of shallow impurities was clearly identified. Carbon and zinc were found to be the major shallow acceptors in most samples. At very low V/III ratios, carbon was the most dominant acceptor. The carbon concentration diminishes with an increasing ratio and the amount of zinc becomes more significant.