The effect of substrate orientation upon the material properties of molecular beam epitaxy grown carbon doped GaAs on (100), (111A), and (111B) orientated substrates with doping levels varying from 10(17) to 10(20) cm(-3) is reported. Van der Pauw Hall measurements show that all samples are p-type regardless of substrate orientation. Photoluminescence (PL) measurements at atmospheric pressure show the band gap shrinkage and the broadening of the PL peaks as the doping concentration is increased. The broad PL peak is believed to arise from a combination of transitions from the conduction band (CB) to heavy hole valence band and CB to light hole valence band. Heavily doped GaAs:C samples show a series of sharp lines at high pressures that are believed to be due to transitions involving near donor-acceptor pairs.