Thermal and reduction-oxidation stability of substituted LaMn1-yCoyO3 perovskite-type oxides (0.0 <= y(Co) <= 1.0) prepared by the citrate route have been studied by means of surface area, X-ray diffraction, FTIR spectroscopy and magnetic properties. The perovskite orthorhombic structure is found for y(Co) <= 0.5, with the exception of y(Co) = 0.1, which corresponds better to rhombohedral LaMnO3.15. For y(Co) > 0.5 the diffraction profiles are quite similar to the cobaltite's rhomboliedral structure. Magnetic is field studies (ZFC-FC) reveal that, for y(Co) <= 0.50, the system presents an anti ferromagnetic canted-like ordering of the Mn/Co sublattice, in which the presence of divalent Co ion creates Mn3+-Mn4+ pairs that interact ferromagnetically through the oxygen orbital. This interpretation is confirmed by the magnetization loops, in which the magnetic moment increases when substituting Mn for Co. Therefore. the general trend is: for y(Co) <= 0.5, the Co ions are inserted in the manganite structure and for y(Co) > 0.5, the Mn ions are inserted in cobaltite structure. The enhancement of the ferromagnetic properties and the thermal stability against reduction for y(Co) = 0.5 is attributed to optimized CO2+ - Mn4+ interactions. (C) 2008 Elsevier Ltd. All rights reserved.