La1-xSrxNiO3 (x = 0.00-0.20) mixed oxides have been prepared and tested in the combustion of methane. X-ray diffraction (XRD) profiles revealed the presence of a single perovskite structure for substitutions 0 less than or equal to x less than or equal to 0.05, whereas a mixture of the major perovskite phase and minor NiO, SrO and SrCO3 phases were observed for, substitutions x greater than or equal to 0.10. Temperature-programmed reduction (TPR) profiles showed: (i) a first reduction step at 520-650, due to reduction of Ni3+ to Ni2+, in which the perovskite structure is preserved although distorted by the oxygen vacancies; (ii) an intermediate reduction step at 589-650 K associated to reduction of the segregated NiO phase; (iii) a final step at 720-800 K assigned to the complete reduction of the Ni2+-containing phases into La2O3 and Ni-0. The partial substitution of Sr by La alters the oxide stoichiometry, yielding a mixture of Ni-II/Ni-III oxidation states and oxygen vacancies, together with surface enrichment of Sr-containing phases. A close relationship has been found between substitution degree (x) and the oxygen non-stoichiometry. The highest intrinsic activity for the CH4 combustion in sample x = 0.10 is associated not only with the largest proportion of oxygen non-stoichiometry but also with the lowest Sr segregation on the surface.