The effect of rare-earth ions (La, Pr, Nd, and Gd) in ACoO3 perovskites on the thermal behavior and on the catalytic activity for methane oxidation has been studied. The thermal reduction, as investigated by oxygen desorption analysis, revealed for all perovskites one evolution step occurring above 1100 K. Transmission electron micrographs at the beginning of the desorption step revealed the presence of superstructures, which are attributed to the formation of oxygen-deficient domains (ACoO2.5 less-than-or-equal-to ACoO3-x less-than-or-equal-to ACoO3) adopting perovskite-related brownmillerite-type structures. A model for the sequence of the structural changes of the perovskites during the reduction is proposed. The amount of oxygen which evolved above 1100 K increased with decreasing size of the lanthanide ion. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600-1200 K. The activities at 830 K, expressed as reaction rates referred to the BET surface area, increased in the order PrCoO3 much less than LaCoO3 < NdCoO3 < GdCoO3. However, no significant influence of the A -site cations on the catalytic activity was found, with exception of praseodymium. PrCoO3 showed a 30 times lower activity than the other samples. The catalytic activity of ACoO3 perovskites can be markedly affected by the presence of Co3O4 impurity in the perovskites.