Diffusion of pure components (hydrogen (H 2), argon (Ar), krypton (Kr), methane (C1), ethane (C2), propane (C3), n-butane ( nC4), and n-hexane ( nC6)) in silica nanopores with diameters of 1, 1.5, 2, 3, 4, 5.8, 7.6, and 10 nm were investigated using molecular dynamics (MD). The MaxwellâStefan (MâS) diffusivity ( Ä i , s ) and self-diffusivities ( D i , self , s ) were determined for pore loadings ranging to 10 molecules nm â3. The MD simulations show that zero-loading diffusivity Ä i , s (0) is consistently lower, by up to a factor of 10, than the values anticipated by the classical Knudsen formula; the differences increase with increasing adsorption strength. Only when the adsorption is negligible does the Ä i (0) approach the Knudsen diffusivity value. MD simulations of diffusion in binary mixtures C1âH 2, C1âAr, C1âC2, C1âC3, C1â nC4, C1â nC6, C2â nC4, C2â nC6, and nC4â nC6 in the different pores were also performed to determine the three parameters Ä 1, s , Ä 2, s , and Ä 12, arising in the MâS formulation for binary mixture diffusion. The Ä i , s in the mixture were found to be practically the same as the values obtained for unary diffusion, when compared at the same total pore loading. Also, the Ä i , s of any component was practically the same, irrespective of the partner molecules in the mixture. Furthermore the intermolecular species interaction parameter Ä 12, could be identified with the binary MâS diffusivity in a fluid mixture at the same concentration as within the silica nanopore. The obtained results underline the overwhelming advantages of the MâS theory for mixture diffusion in nanopores. Our study underlines the limitations of the commonly used dusty-gas approach to pore diffusion in which Knudsen and surface diffusion mechanisms are considered to be additive.