Polycrystalline diamond films exhibiting (100) oriented surfaces have been subject to a low pressure hydrogen plasma for durations up to 20 h. The topography of spatially defined 20 x 20 mu m areas of the samples were imaged by atomic force microscopy at intervals during the plasma exposure. The mean surface roughness of individual (100) crystallites decreased from ca. 2.4 nm to < 1 nm over the period and was independent of the twist and tilt angles of the crystallite. Whilst small hillock growth features were etched completely by the plasma treatment, there was no evidence for etch pits evident in similar experiments carried out with (100) natural diamond. Very low lateral etch rates of the (100) crystallites of 28 +/- 4 nm/h were measured for crystallites bounded by (111) planes. High resolution XPS analysis of the C(1s) and O(1s) transitions of the same samples showed that the surface graphitic phase, present in the as-prepared samples, was removed to below detectable limits. The surface oxygen content was reduced from around 9-10% to ca. 3% after prolonged plasma exposure. The C(1s) and O(1s) band contours revealed the presence of oxygen in the form of ether and carbonyl functional groups. The ether : carbonyl: areal density ratio on (100) crystallites decreased only slightly from 83 : 17 to 64 : 37 after 20 h of plasma treatment based on fitting of the O(1s) band envelope. Etching products arising from the plasma interaction with the diamond surface were not detected by either optical emission spectroscopy or mass spectrometry.