Pt single atom catalysts anchored on the surface of anatase TiO2 have recently been shown to be stable through oxidation and reduction conditions and exhibit higher catalytic activity in CO oxidation than Pt metallic clusters. In this work we unravel the atomistic nature of the isolated Pt-iso, species on anatase TiO2 thanks to the combination of CO probe molecule Fourier Transform Infrared (FTIR) and temperature programmed desorption (TPD) experiments with an extensive set of Density Functional Theory (DFT) calculations. We used the stretching frequencies and adsorption energies of CO bound to Pt as a fingerprint of the specific structure of the stable Pt-iso, species. These consist of Pt atoms bound to two excess O atoms on the surface (PtO2). The excess oxygen atoms on the surface arise from the formation of surface hydroxyl groups, and provide a solid anchor to the oxide support, which explains the high thermal stability of the single-atom Pt catalyst through oxidative and reductive treatments. Beside the single atom Pt species, also models of metallic, Pt-metal, and oxidized, Pt-ox, sub-nanometer clusters have been considered. Comparisons between characteristics of Pt-iso species and sub-nanometer Pt-metol and Pt-ox clusters demonstrate that the combination of CO vibrational frequency and adsorption energy can be an effective approach to differentiate these species. (C) 2018 Elsevier Inc. All rights reserved.