Unintentionally doped epitaxial GaAs grown by low-pressure metalorganic vapour phase epitaxy has been characterized using both conventional electrical deep-level transient spectroscopy (DLTS) and optical DLTS (ODLTS). From this study it is shown that the choice of substrate material is important when characterizing material to be used for process-induced defect characterization. Material grown on silicon-doped n+ (1018 cm-3) GaAs substrate material has an electron defect, a copper defect and two hole traps that have not yet been reported. The three hole traps, presumably introduced by the n+ substrate, compensate the material, causing a reduction in the carrier concentration from 2*1014 cm-3 when the epitaxial layer is grown on semi-insulating (SI) substrate material to 4*1013 cm-3 when grown on n+ (1018 cm-3) substrate material.