Extensive research has been devoted to the assignment of IR bands of CO adsorbed on Pt nanoparticles, which are widely used in heterogeneous and electrocatalysis (e.g. fuel cells). In contrast to single crystal studies, the assignment of CO adsorption to the nanoparticle structure is still controversial. Here we present a case study where we assign CO adsorption bands to the structure of Platinum nanoparticles with a given size distribution. Using a special diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell allows to achieve high quality data under in-situ conditions. Temperature dependent CO adsorption spectra are resolved into three bands which depend on the applied flow and pretreatment conditions. Our calculations using Density Functional Theory (DFT) can mimic the experimental findings and link these bands to the particle structure. By explicitly calculating the IR spectra of CO/Pt nanoparticles of different sizes we show that the IR bands are due to a combined size and site effect. For fully covered small nanoparticles the IR bands are attributed to all binding sites. For larger nanoparticles the dominant contribution is related to {111} facets but the other bands are still site independent. Here we provide a tool to assign CO adsorption bands on Platinum nanoparticles with a given size distribution. This can be related to the structure-acitvity relationship which is required for a tailored catalyst design. (C) 2016 Elsevier B.V. All rights reserved.