The nonreactive coadsorption of CO and deuterium was investigated with temperature programmed desorption and Fourier transform infrared reflection-absorption spectroscopy. Both chemisorbed CO and deuterium adatoms block the adsorption of one another, and no replacement of either of the adsorbates by the other could be detected. Both the peak position and the linewidth of the CO stretching frequency change upon coadsorption and, together with the thermal desorption spectra, indicate the formation of separate islands of the two adsorbates on the surface.