Adsorption of D on highly oriented pyrolytic graphite (HOPG) surfaces was studied with scanning tunnelling microscopy (STM). At low coverages atomic resolution of D on the surface was achieved. Below 0.1% coverage only singly adsorbed D were found. At about 1% pairs of adsorbed D dominate the surface. Three pair types were identified with D bound to opposite corners of a single C-6 ring or of two adjacent C6 rings. At increasing D coverage clusters of adsorbed D develop. Approaching saturation the whole surface appears to be modified by D but STM patterns display no details. (c) 2006 Elsevier B.V. All rights reserved.