Abstract Oxygen adsorption on a Mo(1 1 2) has been studied using HREELS, LEED and TPD. Oxygen molecules dissociatively adsorb to occupy quasi-threefold hollow site, long bridge site and atop site on a clean Mo(1 1 2) at 100 K. After annealing this surface to 300 K, a loss peak corresponding to the oxygen atoms at atop sites disappears, changing the adsorption sites into quasi-threefold and long bridge sites. Further annealing to 600 K resulted in the change of the location of oxygen atoms from long bridge site to quasi-threefold hollow site, where the surface shows p(1×2) LEED subspots, indicating that the oxygen atoms in the Mo(1 1 2)–p(1×2)-O surface occupy quasi-threefold hollow sites. Oxygen adsorption on the Mo(1 1 2)–p(1×2)-O surface was also investigated and it was found that oxygen molecules associatively adsorb on the surface to give bridged peroxo species, which dissociates at 200 K to yield oxygen atoms at atop sites. The atop oxygen atoms remain on the surface after annealing to higher temperatures in contrast to the adsorption on clean Mo(1 1 2). Further annealing to 800 K resulted in a disordered surface associated with a molybdenum oxide (Mo x O y ) layer.