The structure, thermal stability, and reducibility in H-2 of sulfate species formed in MgO-promoted perovskites, LaMn0.5Mg0.5O3.17 MgO, have been studied by infrared spectroscopy, temperature-programmed desorption, and temperature-programmed reduction-mass spectrometry. Sulfation at 1073 K generates on the MgO phase both bulk and subsurface sulfates, whereas the latter species are the major product of sulfation at room temperature. In contrast with high-surface MgO samples (where sulfate species of covalent nature are found), the surface species closely resemble those in the bulk. Bulk sulfates are more resistant to thermal treatments than surface sulfates. A transformation of bulk sulfates at higher temperature into sulfides is suggested by IR spectroscopy and proved by chemical analysis. SO2, SO, and H2S are the products of sulfate reduction.