Mechanochemical synthesis of La1-xSrxFe3-y (0 less than or equal to X less than or equal to 1) perovskites was carried out from simple oxides. Undoped lanthanum ferrite samples calcined at 900 and 1100degreesC for 4 h are nearly single-phase systems, while some substituted ones (samples with 0.3 less than or equal to x less than or equal to 0.8 calcined at 900degreesC and samples with x = 0.3, x = 0.4 and x = 0.8 calcined at 1100degreesC) are two-phase systems consisting of orthorhombic perovskite La1-xSrxFeO3-y (A) and cubic or tetragonal perovskite SrzLa1-zFeO3-y (B) phases. In CO and CH4 oxidation processes, the specific catalytic activity (SCA) of samples calcined at 1100degreesC varies non-monotonously with the Sr content. In CO oxidation, SCA reaches a maximum in the range of La substitution by Sr where phase transition occurs and samples are comprised of two coexisting phases (x = 0.3 and 0.8). In CH4 oxidation, SCA decreases with the Sr content. (C) 2004 Kluwer Academic Publishers.