An essential property of combustion catalysts is long-term (> 8000 h) stability at high temperatures in an environment (similar to1 atm of both oxygen and water vapor) that aggressively promotes sintering of the supporting oxide and coarsening of the active component. Extrapolation of accelerated coarsening rate measurements, determined from shorter exposures at higher temperatures, can be made with more confidence if the physical processes of the coarsening and sintering processes were well understood. The current work examines in detail the coarsening of a high-weight-loaded palladium catalyst supported by silica-stabilized alumina at 900 degreesC in such an aggressive environment. The results of this investigation showed that the Pd particle size distribution was consistently log-normal for time periods from 100 to 4000 h, the mean particle growth rate was roughly inverse second-order in mean particle diameter, and the support not only sintered but also underwent phase transformation. The results implicate both coalescence and Ostwald ripening as important coarsening processes.