A series of perovskite-like oxide in which the A-site cation of the precursor perovskite, LaRu0.8M0.2O3, was partially or totally substituted by calcium, samarium and neodymium have been used to produce in situ nanoparticles of Ru(Ni) well dispersed on a stable support for the carbon dioxide reforming of methane. Perovskites of the type Ln(x)Ca(1 - x)Ru(0.8)Ni(0.2)O(3) (Ln = La3+, Sm3+, Nd3+) were synthesized as catalysts precursors. The reduced solids of nominal composition (Ru,Ni)/CaO and/or La2O3, Sm2O3, Nd2O3, were used as catalysts. The La1 - xCaxRu0.8Ni0.2O3 series showed a well-defined perovskite structure with surface areas between 3 and 10 m(2)/g. However, when lanthanum was replaced by samarium and neodymium, the presence of pyrochlore structures, together with the perovskites, were obtained. After reduction Ru(Ni) crystallites size between 9 and 17 nm were produced. The substitution of La by cations of smaller ionic radii (Ca, Nd, Sm) decrease the stability of the perovskites and lower their reduction temperature. Among the calcium series, La0.8Ca0.2Ru0.8Ni0.2O3 and La0.5Ca0.5Ru0.8Ni0.2O3, proved to be the most active catalysts with the highest selectivity to CO. While samarium-containing perovskite was the best among the lanthanide series. Correlations between the effect of partial or total substitution of A-site cations of the precursor perovskite and the catalytic activity and stability of in situ formed nickel and ruthenium particles were established. (C) 2003 Published by Elsevier B.V.