The activities of three-way catalysts containing various Pt and Rh loadings have been tested in a laboratory reactor. These tests were performed on the catalysts both when they were fresh and following aging in engine exhaust. Increasing the Pt and Rh loadings from 0.1 wt % and 0.01 wt %, respectively, to 0.2 wt % and 0.02 wt %, respectively, resulted in improved catalyst lightoff and improved warmed-up performances during A/F ratio cycling for both the fresh and the aged catalysts. Increasing only the Rh loading of these bimetallic catalysts resulted in a greater improvement in the lightoff and warmed-up NO conversion efficiencies than resulted from increasing only the Pt loading. Increasing the Pt loading, however, resulted in a greater improvement in the warmed-up CO conversion efficiencies than did increasing the Rh loading. Increasing either the Pt or the Rh loading resulted in similar improvements in the warmed-up HC conversion efficiencies.