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MnBiAl films are of considerable current interest for possible magneto-optical applications because of their perpendicular anisotropy, large polar Kerr rotation, reduced grain size, and structural stability. We report here experimental and theoretical studies of the effect of Al alloying on the electronic structure and magneto-optical properties of MnBi. Our measured spectral dependencies of the polar Kerr rotation in the two systems are similar. We carried out relativistic self-consistent spin-polarized electronic structure calculations on MnBi and MnBiAl, and the calculated densities of states are in good agreement with available x-ray photoemission spectroscopy data for both materials. The spin magnetic moment of Mn increases and the overall orbital moment decreases with the addition of Al to MnBi. The latter effect implies the lowering of spin-orbit interactions upon alloying MnBi with Al, and the two effects together suggest that the spectral dependencies of the Kerr rotation should be similar, as is observed experimentally.