GaAs epitaxial layers have been grown on semi-insulating GaAs substrates using the technique of metalorganic chemical-vapour deposition. The organometallic compounds trimethylgallium and trimethylgallium–trimethylarsenic adduct were used as source reagents for gallium, whereas arsine and trimethylarsenic were used as sources for arsenic. Photoluminescence measurements at 8 K indicated that carbon was present in the layers as the dominant background acceptor, in most cases, and to a lesser extent copper (Cu) and manganese (Mn) acceptors were also present. Secondary-ion mass spectroscopy (SIMS) analysis confirmed the presence of these acceptors. Silicon was also identified by SIMS in most layers. The concentration of Cu and Mn was correlated with the starting substrate and with the deposition parameters (growth rate, deposition temperature). It was also found that their concentration in the epitaxial layers could be reduced by a careful heat-treatment and chemical etch prior to epitaxial growth.