The incorporation of impurities in GaAs epitaxial layers grown from trimethyl gallium (TMG) and AsH3has been studied in detail by varying a large number of growth parameters. These include the V/III ratio, temperature, the axial position in the reactor, gas sources, substrate and susceptor material, carrier gas, substrate misorientation, and the crystallographic orientation of the substrate. As main characterization techniques photoluminescence and Hall–van der Pauw measurements have been used. Donor and acceptor concentrations in the layers have been found to vary not only with temperature and V/III ratio, but also with the axial position in the reactor, giving rise top/ntransitions and maxima in the carrier mobility. The V/III ratio is shown to be effectively constant for larger axial distances in the cell. Highly doped substrates have been found to give rise to outdiffusion of defect complexes into the layers. The main acceptor impurities found in this work are zinc, silicon, and carbon. They are shown to originate from the TMG gas source, the hot quartz parts in the cell, and the TMG growth component, respectively. Incorporation of these elements appears to be orientation, and in the case of carbon also misorientation, dependent. The results for {001} and misoriented {001} crystals are discussed on the basis of two models for impurity incorporation: CAsacceptors are concluded to be incorporated by a trapping process at growth steps, whereas ZnGa, SiAs, and residual donors most probably incorporate via equilibrium processes. The conclusions from a study of the influence of the misorientation are especially important for this interpretation.