The effects of Al as an isoelectronic dopant have been investigated on the deep level defects in GaAs grown by molecular‐beam epitaxy (MBE), using deep level transient spectroscopy. Two different compositions containing 0.1% Al (Al0.001Ga0.999As) and 1% Al (Al0.01Ga0.99As) have been studied. At least nine different deep levels have been detected. Their detailed characteristics consisting of emission rate signatures, capture cross sections, concentrations, and junction depth profiles have been determined. The deep levels observed have been compared with the M levels normally found in MBEn‐GaAs. The emission rates of deep levels have been found to shift to higher values with decrease in Al concentration. This fact has been attributed to lattice strain and random alloy effects. The relative concentrations of deep levels are seen to undergo large changes as the Al concentration is increased from 0.1% to 1%. Al doping upto 0.1% does not seem to reduce the deep level concentration, unlike the case of other isovalent dopants such as In and Sb in MBE GaAs. Further increase in the Al doping to 1% is found to lead to a pronounced increase in the overall deep level concentration. These data along with the other measured characteristics are interpreted in terms of the possible models for the various defects. ©1995 American Institute of Physics.