The aging response of a SiC particulate reinforced powder metallurgy aluminum composite was examined as a function of particle size and volume fraction. The addition of SiC particles ranging in size from 24 to 142 μm at 9 vol pct had no effect on the aging kinetics of the composites. Acceleration of the aging behavior or inhibition of the initial stage of the age-hardening process was observed at 18 and 27 vol pct. The accelerated aging kinetics were consistent with smaller particles creating larger thermal misfit dislocation densities. In addition, it was shown that different combinations of ceramic particle size and volume fraction lead to similar effects on the aging behavior. Loss of the initial hardening response was attributed to the suppression of Guinier-Preston (GP) zone formation due to the annihilation of excess vacancies at the thermal misfit dislocations. © 1995 The Minerals, Metals & Material Society.