The adsorption of D2O on Pt(111) and the coadsorption with CO have been revisited by quantitative and in situ high-resolution X-ray photoelectron spectroscopy (XPS). During D2O adsorption at 110 K, O 1s spectra clearly show the known formation of a bilayer and multilayers. As indicated by a binding energy change of the multilayer peak in the XP spectra recorded during heating, a structural change in the D2O multilayer at the onset of multilayer desorption (similar to145 K) is observed. Coadsorption of CO and D2O leads to occupation of bridge and on-top sites by CO, with the order in site occupation reversed as compared to the clean surface, as previously observed. In addition, a new CO species is clearly resolved in O 1s spectra, which may be identified with CO on hollow sites. Upon D2O desorption, this new species disappears and the CO site occupation changes to the value observed on the clean Pt(111). Induced by CO coadsorption, partial replacement of D2O from the surface into higher layers is observed, indicated by a reduced desorption temperature of water and an O 1s binding energy shift toward the value of the multilayer signal. Our results suggest an intermixed first layer of CO and D2O molecules; D2O on top of this layer also influences the CO site distribution. On a surface precovered with 0.5 monolayer (ML) of CO, D2O molecules can only adsorb on top of the CO layer and no mixed layer is formed; hence, no site change is observed. The preadsorbed D2O bilayer lowers the initial CO sticking coefficient by a factor of similar to13 as compared to the clean surface; this reduction is larger than previously detected.