LaMn1-xMgxO3. yMgO catalysts (x = 0, 0.2, 0.5, y = 2, 6, 17) were prepared, characterised (X-ray diffraction (XRD), BET, scanning electron microscopy (SEM)-energy dispersion spectroscopy (EDS), transmission electron microscopy (TEM), FTIR, chemical analysis and atomic absorption), tested for high-temperature methane combustion and aged in presence of SO2 to investigate the effect of sulphur on their catalytic activity. Two important roles of MgO have been assessed: (i) MgO acts as promoter for the catalytic activity of fresh LaMn1-xMxO3. yMgO catalysts up to a certain MgO excess (y = 6), (ii) MgO slows down sulphur poisoning of all LaMn(1-x)Mg(x)O3 . yMgO catalysts. LaMn1-xMgxO3. 17MgO catalysts show the highest resistance to sulphur poisoning and their deactivation is appreciable only after 32 days of exposure to 200 ppmv SO2 at 800 degreesC. They, then, can be regenerated completely by aqueous NH3 washing. Sulphate formation on both perovskite and the MgO phase is proposed to be responsible of catalyst deactivation. An explanation of the nature of these sulphates, also based on FTIR investigation, is given.