A series of unintentionally doped Ga1−xAlxAs epitaxial layers grown by molecular beam epitaxy, having various Al compositions (0.15, 0.18, and 0.29), have been Li diffused at 300 °C. Capacitance–voltage techniques show that the initialn‐type doping concentrations (of the order of 1016cm−3) increase after diffusion by typically a factor 5 to 10 demonstrating that isolated Li interstitials do exist and behave as donors. However, secondary‐ion mass spectroscopy measurements show that the material contains 1019–1020Li cm−3, which indicates that a large fraction of the Li impurities is not electrically active. Thus Li also produces defects as revealed by the fact that free electrons are frozen in the diffused layers below 77 K. Search for the existence of Li associatedDXcenters deep defects related to the donor impurities has been performed by deep level transient spectroscopy. Only theDXcenters present before diffusion, i.e., associated with residual donor impurities, are detected in the layer of 0.29 Al fraction. This implies that either Li donors do not induce the existence ofDXcenters or electron emission from LiDXcenters occurs below 77 K.