Abstract The direct-inelastic (DI) components of the in-plane angular scattering distributions of CO and Ar along the 〈110〉 azimuth of Cu (110) have been measured. For the clean surface the DI channels of both CO and Ar exhibit scattering which is characteristic of energy exchange perpendicular to the surface via substrate phonon modes. Low coverages of adsorbed sulphur do not perturb the general inelastic scattering behaviour except for a particular scattering condition where there is a significant increase in the average normal momentum transferred to the DI channel. This resonance in energy transfer appears to be mediated by the adsorbed sulphur, and occurs only when the normal energy component of the incident Ar or CO corresponds to 50±2 meV and 50 to 54±2 meV respectively. The resonance disappears as the sulphur coverage increases to produce well ordered overlayers.