Infrared reflection absorption spectroscopy (IRAS) was used to study the adsorption of carbon monoxide on mixed S and Co over layers on a Mo(110) surface. The results of this study indicate that there is a migration/segregation of the adatoms into 2-D and 3-D islands and that the different domains are distinguishable by the stretching frequency of the adsorbed CO. Clean Mo(110) as well as sulfided Co, (8X2)Co, p(2X2)S, and c(2X2)S domains on the Mo(110) surface are all observable. In addition, sulfur was seen to act as both a site blocker and electron-withdrawing agent on these surfaces. Finally, data obtained in this study suggest that the active adsorption site in a Co-MoS2 catalyst could be a Co-promoted Mo-S site.