We investigated changes in the Pt-Pt bond distance, particle size, crystallinity, and coordination of Pt nanopartides as a function of particle size (1-3 nm) and adsorbate (H-2, CO) using synchrotron radiation pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) measurements. The similar to 1 nm Pt nanopartides showed a Pt-Pt bond distance contraction of similar to 1.4%. The adsorption of H-2 and CO at room temperature relaxed the Pt-Pt bond distance contraction to a value close to that of bulk fcc Pt. The adsorption of H-2 improved the crystallinity of the small Pt nanopartides. However, CO adsorption generated a more disordered fcc structure for the 1-3 nm Pt nanopartides compared to the H-2 adsorption Pt nanopartides. In situ XANES measurements revealed that this disorder results from the electron back-donation of the Pt nanoparticles to CO, leading to a higher degree of rehybridization of the metal orbitals in the Pt-adsorbate system.