The results of experimental investigation of free-molecular gas flow in channels with an atomically pure and adsorbed surface are presented. The data obtained made it possible to formulate the basic principles which reflect the character of the effect of the chemical composition of a surface on the gas-dynamic conductivity of the channel. Qualitative interpretation of the obtained experimental results is given within the framework of the existing formalism in the description of the scattering of molecular beams by the surface of a solid body. The diffusion factors ε have been calculated for different surfaces and gases from experimental data. Satisfactory agreement between numerical simulation by the Monte Carlo method and the experimental data is seen.