Pt/Ce alloys treated with O-2, N2O, or CO/H-2 yield Pt/ceria catalysts that are active for CO oxidation. The most active catalysts are produced by N2O treatment of Pt(3)C(e)7 at 720 K, which leads to the highest dispersion of both the Pt and ceria phases. This reflects the smaller exotherm, which occurs with N2O, apparently a critical requirement for production of a highly active catalyst. The performance of these Pt3Ce7-derived catalysts is comparable with that of the best, recently announced, materials prepared by wet chemical techniques. In all cases, most of the active material is invisible to X-ray diffraction and the catalysts exhibit very low CO-titratable Pt area (<0.05 CO/Pt). Prereduction in H-2 at 570 K greatly enhances the reactivity of a given catalyst. CO TPR shows that highest activity correlates with a reduction state at similar to 510 K, suggesting metal-promoted CO oxidation by the ceria. It seems possible that this high activity is associated with ceria spillover onto Pt, the spillover being facilitated by atomic-level mixing of Pt and Ce in the alloy precursor.