La2CrO6 (Cr6), LaCrO3 (Cr3), LaCrO3-La2CrO6 (Cr6-Cr3) and LaCrO3-La2O3 (Cr3-L) catalysts were synthesised and investigated with in situ X-ray diffraction (ISXRD) during methane catalytic combustion in order to characterise the solid phases present under reactants and to determine the effect of chromium oxidation state on the catalytic activity. Methane conversion was evaluated over a temperature range of 300-800 A degrees C, using oxygen-to-methane ratio of 4 and GHSV of 8,000 h(-1). The TPR provided information about the oxygen depletion temperatures characteristic of lattice oxygen mobility in the samples and ISXRD results evidenced a cooperative effect of Cr3 and Cr6 phases at low temperatures (< 725 A degrees C) and of Cr3 and LaCrO4 (Cr4) phases at high temperatures (> 750 A degrees C). The relative phase composition determined the oxygen activation capability and hence the corresponding activity for the oxidation of methane. It was observed that while direct and back Cr6 a dagger" Cr4 transition temperatures were unaffected by Cr6 content in the samples, the methane conversion was strongly modified. This suggests that Cr3+/Cr6+ and Cr3+/Cr5+ species involved substantial modification of the surface chemistry which affected the catalytic activity. These results provide the first direct evidence of the presence of Cr4 metastable phase during methane combustion over La-Cr-O catalysts.