We study magnetic circular dichroism of absorption (MCDA) of LT AlxGa1−xAs as a function of aluminum content. The MCDA spectrum of LT AlxGa1−xAs is distinctly different from the MCDA spectrum of LT GaAs, which has one paramagnetic and one diamagnetic peak at 0.95 eV and 1.18 eV, respectively. As the aluminum content increases, the spectrum of LT AlxGa1−xAs is dominated by a diamagnetic peak similar to the 1.18eV peak of the EL20-like defects in LT GaAs. However, the peak shifts to higher energies as x increases. The photoquenching and temperature dependence of this peak indicates an association with the EL2 defect. The paramagnetic peak observed in LT GaAs also shifts to higher energies but faster and eventually merges with the diamagnetic peak as the Al content increases. The study of the MCDA spectrum of LT AlxGa1−xAs as a function of aluminum content allows a better understanding of the MCDA phenomena of LT AlxGa1−xAs and LT GaAs, as well as the EL2-related transitions in bulk semi-insulating GaAs.