Single atoms of platinum group metals on CeO2represent a potential approach to lower precious metal requirements for automobile exhaust treatment catalysts. Here we show the dynamic evolution of two types of single-atom Pt (Pt1) on CeO2, i.e., adsorbed Pt1in Pt/CeO2and square planar Pt1in PtATCeO2, fabricated at 500 °C and by atom-trapping method at 800 °C, respectively. Adsorbed Pt1in Pt/CeO2is mobile with the in situ formation of few-atom Pt clusters during CO oxidation, contributing to high reactivity with near-zero reaction order in CO. In contrast, square planar Pt1in PtATCeO2is strongly anchored to the support during CO oxidation leading to relatively low reactivity with a positive reaction order in CO. Reduction of both Pt/CeO2and PtATCeO2in CO transforms Pt1to Pt nanoparticles. However, both catalysts retain the memory of their initial Pt1state after reoxidative treatments, which illustrates the importance of the initial single-atom structure in practical applications.