Pt-Rh three-way catalysts (TWCs) of two formulations were examined by electron probe microanalysis (EPMA) and cross sectional transmission electron microscopy (TEM), in order to study the effect of accelerated aging on microstructure. Both formulations have Pt and Rh applied in separate washcoat layers, Rh in a zirconia-rich outer layer and Pt in an alumina-rich inner layer(s). Microstructural analyses show that in spite of the initial separation, the precious metals inter-diffuse between the washcoat layers and alloy during aging. Following dynamometer aging the two formulations exhibited significant differences in the precious metal (PM) particle size distributions and the correlation between PM particle size and composition. These differences are attributed to the increased transport of Pt due to the presence of ceria and zirconia in the alumina-rich inner washcoat of one formulation, compared to other formulation. The presence or absence of porosity > 1 mu m in diameter and differences in initial Pt distributions in the inner washcoat were also noted as possibly contributing to the PM particle growth. The transition alumina of one formulation, containing Ba, partially transformed to corundum during aging. The other formulation utilized a La-stabilize dalumina which did not transform to corundum after aging.