The effect of surface material and roughness on the conductance of a channel at molecular flow was measured for N-2, Ar, He, and H-2. The experimental results were compared with the results of Monte Carlo calculations. The channel consisted of two parallel disks with different surfaces including polished stainless steel (SS), roughly polished SS, quartz, Ti, Cu, Al, alumina with smooth surface, alumina with rough surface, Au coating, Pt coating, and diamondlike carbon (DLC) coating. The conductances of the experimental channel depended on the slope of the surface roughness, rather than the arithmetic surface roughness (Ra) or the material. In the case of smooth surfaces, such as polished SS, quartz, and DLC coating, the experimental results for N-2, Ar, and H-2 show good agreement with the calculation, assuming diffuse reflection. The momentum accommodation coefficients on these surfaces for N-2, Ar, and H-2 were estimated to be in the range from 0.98 to 1.00, based on the assumption that specular reflection was simply included in diffuse reflection. The conductance for He, however, was 3%-5% larger than that of the calculation. The momentum accommodation coefficient for He was similarly estimated to be in the range from 0.95 to 0.97. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3306024]