The oxidation kinetics of spherically shaped palladium (Pd) powders was investigated over the temperature range between 350 and 800 degrees C. Two types of Pd powders differing from each other in particle sizes were used in this study, 0.30 and 0.85 mu m in diameter. The fraction of total oxide formed was measured in situ by means of a thermobalance method. In this system, the volume of product palladium oxide (PdO) increases significantly vs. the volume of reactant Pd, and consequently the experimental data were explained well by Valensi-Carter equation. Apparent activation energies of oxidation for the two types of Pd powders were both estimated to be 140 kJ/mol.