The photolytic reactions of AlX (X = F, Cl, and Br) with O-2 in solid argon matrixes are shown to yield the peroxo species XAlO2, all exhibiting C-2v symmetry. The species were identified and characterized by means of IR spectroscopy allied with quantum mechanical calculations. In addition to singlet XAlO2 as the main product of the reaction of AlX, the experiments give clear evidence for the formation of XAl(mu-O)(2)AlX, by the reaction of the dimer (AlX)(2), which is also known to be present in the matrixes upon deposition. Finally, weak IR absorptions were tentatively assigned to XAlO2 in its triplet electronic state. According to our calculations, the singlet-triplet gap amounts to about 40 kJ mol(-1) for all species. The properties of the peroxide species invite comparison with previously investigated dioxygen complexes, as well as the superoxide species XAlOO and various possible products of the reaction of (AlX)(2) dimers.