Platinum nanoparticles dispersed on nanosized ceria areactive for CO oxidation at room temperature after hydrogen pretreatment.High angular annular dark field scanning transmission electron microscopy(HAADF-STEM) analysis of the reduced catalyst shows spreading of the 1nm sized platinum particles under the electron beam, characteristic for atwo-dimensional strong metal−support interaction. In situ X-ray absorptionfluorescence spectroscopy (XAFS) reveals a Pt−O distance of 2.1 Å, whichis significantly longer than the Pt−O distance in PtO2 (2.0 Å). Thiselongated Pt−O distance can be related to interaction of the platinumspecies with cerium oxide in the form of a low-temperature active species−support interaction. These findings contribute to the general understandingof catalytic systems operating at low temperature.