Concentration dependent, binary, coupled, diffusional mass transport, through dense membrane, was investigated. A quasi-analytical approach is given in order to calculate the concentration distribution in the membrane and the mass transfer rates in the case of a single component as well as of binary, coupled component diffusion transport. Using them, the effect of the tube-side external mass transfer coefficient, of the cylindrical space co-ordinate, of the feed concentration on the mass transfer rate was discussed. Both properties can be expressed by closed mathematical equations independently of the concentration dependency function of diffusion coefficients. The essential of this approach is that the composite, inhomogeneous or homogeneous media are divided into N sub-layers with constant diffusion coefficients. The differential mass balance equations with constant parameters were solved for every sub-layer with suitable boundary conditions. The coupled binary diffusion of toluene-water mixture was described by means of Flory-Huggins theory as an example. The widely used Maxwell-Stefan theory can also be similarly applied by the model presented. (c) 2005 Elsevier B.V. All rights reserved.