Gas determinations, carried out by vacuum-degassing procedures, with zone-melted single-crystalline tungsten and molybdenum, show that the oxygen, nitrogen and hydrogen contents of etched crystals increase with the surface-to-mass ratio. This result indicates that concentrations determined by analysis are decisively influenced by surface gases. This effect was most marked with oxygen. On etched tungsten samples the oxygen surface concentration amounted to 2 μg/cm 2 geometrical surface. A short annealing at 800 °C in a hydrogen stream decreased the amount of surface gas without removing it completely. The residual oxygen surface content on tungsten was estimated to be 1 μg/cm 2, which can be explained by a chemisorption layer of atomic dimension. The volume concentration of oxygen could be estimated by extrapolating the analytically determined content to zero surface area. In this way the volume content was found to lie at or below 0.5 p.p.m. O 2 in tungsten single crystals which showed 2–3 p.p.m. O 2 by analysis.