Abstract The overlayer growth of cobalt on Mo(110) and the adsorption of CO and deuterium on Co/Mo(110) have been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and temperature-programmed desorption (TPD). Co grows layer by layer on Mo(110) at a sample temperature of 115 K. Annealing multilayers of Co on Mo(110) to > 400 K results in the formation of three-dimensional (3D) Co clusters. The TPD spectra of Co from Mo(110) reveal two desorption peaks, β 1 and β 2, with activation energies of 82 and 85 kcal/mol, corresponding to desorption from 3D Co clusters and the first uniform Co overlayer, respectively. Submonolayer Co( < 0.75 ML, where 1 ML corresponds to the atomic density of the Co(0001) surface) is believed to form a pseudomorphic layer on Mo(110) upon annealing to 400 K. Co coverages from 0.75 to multilayers, annealed to 500 K, exhibit the following LEED patterns: a 9 × 2 structure, a hexagonal structure, and a 2 × 2 structure. The 9 × 2 and hexagonal structures are interpreted to correspond to a distorted lattice of Co(0001) and to 3D Co clusters on Mo(110), respectively. The TPD spectra of CO or D 2 on Co overlayers show desorption features with lower activation energies than the analogous desorption features from bulk Co(0001).