To reveal the effect of ceria particle size on dispersion and structure of supported Pt catalysts during preparation, activation, and reaction testing, a unique CeO 2 / Al 2 O 3 support (CAÀ c) with smaller CeO 2 particle size and more surface defects was prepared using a colloidal CeO 2 precursor, comparing with a conventional CeO 2 /Al 2 O 3 support (CAÀ n) using cerium nitrate as precursor. More atomically dispersed Pt and abundant PtÀ OÀ Ce structures were observed in the Pt/CAÀ c catalyst than in the Pt/CAÀ n catalyst. Both parent catalysts received significant enhancement on their catalytic CO oxidation activities if activated by 10% hydrogen at 400°C before reaction. Between the two representative catalysts, the extent of activity enhancement upon activation was more pronounced for Pt/CAÀ c. We found that smaller Pt clusters with more active ionic Pt sites were generated on the activated Pt/CAÀ c catalyst, while agglomerated larger Pt particles with more metallic sites were formed on the activated Pt/CAÀ n. The facile formation of Ce 3 + was also indicative of more active metal-support interfaces in the activated Pt/CAÀ c catalyst. These results highlight the importance of regulating ceria support particles to enable a controlled anchoring and subsequent activation of Pt single atoms for low-temperature CO oxidation reaction.