A series of Pd/Al2O3 catalysts with a wide range of mean Pd particle sizes (ca. 2-30 nm in diameter) was prepared by using various precursors (H2PdCl4, Pd(NO3)(2) and Pd(AcAc)(2)) and pre-treatments. The mean particle size of reduced samples was determined by H-2 chemisorption. The catalytic activity in methane oxidation under lean burn conditions was measured. The oxidation of reduced samples was studied at 300 degrees C. The extent of oxidation was found to decrease with increasing mean particle size. While small particles (< 5 nm) oxidised very rapidly, the oxidation of large particles (ca. > 15 nm) proceeded via a two-step process, being first fast and then slow. The decomposition of oxide species was studied by temperature-programmed experiments under vacuum. Two distinct oxidised species with different stability were evidenced depending on the particle size. Oxidised species in larger particles were found of lower stability than in smaller ones. A correlation between the existence of distinct types of oxide species and catalytic properties in methane oxidation was discussed. (c) 2005 Elsevier B.V. All rights reserved.