A 1 wt.-% Pd/gamma-Al2O3 catalyst was prepared (38% dispersion) and sintered under a flow of 3 vol.% O-2 in N-2 at 900 degrees C (9% dispersion). The temperature-programmed oxidation of CO, C3H6, C3H8 and CH4 in substoichiometry of O-2 was carried out on the fresh and, in some cases, on the sintered catalyst. The chemical state of palladium in the sintered catalyst was determined by XRD after interruption of the reaction at different temperatures. The temperatures at which a 20% conversion was obtained on the fresh pre-oxidised catalyst were : CO, 210 degrees C < C3H6, 260 degrees C < C3H8, 300 degrees C < CH4, 335 degrees C corresponding to an activity (in mmol h(-1) g(-1)) at 300 degrees C of : CO, 403 > C3H6, 66 > C3H8, 15.3 > CH4, 7.9. Pre-reduction did not change the catalyst activity in CO and in C3H6 oxidation while it decreased the activity by a factor 2.4 for C3H8 and 3.2 for CH4. Except for a definite decrease of activity, the above conclusions remained valid for the sintered catalyst. XRD measurements showed that PdO initially present in the pre-oxidised sample was reduced before the oxidation of CO (< 180 degrees C) and C3H6 (< 210 degrees C) started, which explains why both pre-oxidised and pre-reduced samples had the same activity. In the case of C3H8, the reduction occurred during the reaction and led to a temporary decrease of activity. For CO and C3H8, the Pd degrees structure was clearly identified while a new structure PdO xi (0 < xi much less than 1) was formed with C3H6 This compound has the cfc structure of Pd degrees with a higher lattice parameter (3.990 Angstrom instead of 3.889 Angstrom in Pd degrees).