In membrane distillation (MD) and gas permeation (GP) processes, a general equation given by dusty gas model (DGM) is often used for gas flux through porous media. However, several discrepancies were found in fitting experimental data with the DGM equation. An assumption based on the fact that gas is a compressible viscous fluid is introduced in a modified DGM equation (MDGM). Polytropic equation of state is used to estimate the variation of gas temperature along the pores of membrane. The gas temperature influences its viscosity, which affects its flow resistance through membrane. The MDGM equations for single gases are provided by incorporating polytropic equation of state when integrating the differential DGM flux equation. The MDGM for single gases can be used in modeling the inconsistencies in using DGM in literature and it is verified by published experimental data through fitting the distinguished trends under DGM into one line. The MDGM reveals that the temperature may decrease in membrane pores and exit side, and the phase change of vapor may be one of the inducements for pore wetting in MD. (C) 2011 Elsevier BM. All rights reserved.