Ageing treatments were carried out on LaMn 1 - x Mg x O 3 + δ (x=0, 0.2, 0.5) catalysts for methane combustion to throw light on their deactivation mechanism in the presence of tetrahydrothiophene - used in Italy as odorant of natural gas - or directly in the presence of SO 2 . Various characterisation techniques were used in concert: XRD, BET, TEM, SEM-EDS, FTIR, TPD, chemical analysis and atomic absorption. The effects of treatment temperature and exposure on catalytic activity were evaluated: exposure to 200ppmv SO 2 at 800 o C for 24h causes a rather drastic deactivation of each LaMn 1 - x Mg x O 3 catalyst. Metal sulphate formation is the major responsible for deactivation. However, the extent of poisoning as well as the catalyst regenerability depends on the catalyst composition. The LaMn 0 . 5 Mg 0 . 5 O 3 catalyst can in fact be regenerated completely by water leaching, while LaMn 0 . 8 Mg 0 . 2 O 3 and LaMnO 3 perovskites can be regenerated significantly only by leaching with aqueous NH 3 solutions. An explanation of these results, also based on the values of activation energy for each fresh, poisoned and regenerated catalysts, is suggested and a mechanism for catalyst regeneration by water and NH 4 OH solution is proposed.