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Reduction of Pd° and decomposition of palladium oxide supported on γ-alumina were studied at atmospheric pressure under different atmospheres (H2, CH4, He) over a 4 wt% Pd/Al2O3 catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd° phase was evaluated by in situRaman spectroscopy and in situXRD analysis. Under H2/N2, the reduction of small PdO particles (<5 nm) occurs at room temperature, whereas reduction of larger particles (>5 nm) starts at 100 °C and is achieved at 150 °C. Subsequent oxidation in O2/N2 leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 °C. Under CH4/N2, the small particle reduction occurs between 240 and 250 °C while large particle reduction is fast …