[en] The characterization of the gas transport properties of porous solids is of interest in several fields of science and technology. Many catalysts, adsorbents, soils, graphites and carbons are porous. The gas transport through most porous solids can be well described by the dusty gas model invented by Evans, Watson and Mason. This model includes all modes of gas tranport under steady-state conditions, which are Knudsen diffusion, combined Knudsen/continuum diffusion and continuum diffusion, both for gas pairs with equal and different molecular weights. In the absence of a pressure difference gas transport in a pore system can be described by the combined Knudsen/continuum diffusion coefficient D₁ for component 1 in the pores, the Knudsen diffusion coefficient D_1K in the pores, and the continuum diffusion coefficient D₁₂ for a binary mixture in the pores. The resistance to stationary continuum diffusion of the pores is characterized by a geometrical factor (ε/τ) < 1, where εis that part of the accessible porosity which is actually involved in steady state gas transport. ε/τhas been termed porosity to tortuosity ratio and is defined by D₁₂ = (ε/τ)D₁₂, were D₁₂ is the continuum diffusion coefficient for a binary mixture in free space. The Wicke-Kallenbach method was often used to measure D₁ as function of pressure. D₁₂ and D_1K can be derived from a plot 1/D₁ νs P, and ε/τcan be calculated since D₁₂ is known. D_1K and the volume of dead end pores can be derived from transient measurements of the diffusional flux at low pressures. From D_1K the expression (ε/τ^c) anti l_por may be calculated, which characterizes the pore system for molecular diffusion, where collisions with the pore walls are predominant. (orig.)