The application of the thermogravimetric analysis (TGA) combined with on-line mass-spectrometry of the effluent gas mixture and temperature-programed oxidation (TPO) experiments allowed to demonstrate that significant variation of the carbon content in Pd catalysts occurred during the oscillatory methane oxidation. Together with the visual observation of the color changes due to the periodic variation of the Pd valence, these data indicate a very complicated mechanism associated with the oscillatory behavior, where both Pd oxidation-reduction processes and carbon deposition-removal play a crucial role. Depending on the phase of the oscillatory cycle, three states of metallic Pd with different reactivities were revealed: highly active, freshly reduced, metallic Pd; moderately active metallic Pd modified by carbon; and poorly active metallic Pd with adsorbed oxygen. It was found that hundreds of carbon monolayers could be periodically accumulated in the Pd powder catalyst during the oscillatory cycle. The accumulation of carbon on the Pd catalyst was the reason for the antiphase oscillations of the reaction products CO2 and H2O. (C) 2009 Elsevier Inc. All rights reserved.