In this paper, we develop an expression of Fick's law to be used in the diffusion-solution model when coupling exists. To derive this relationship we have manipulated the generalised Stefan-Maxwell equation to get a more convenient generalised multicomponent Fick's law in which clearly defined diffusivities and coupling coefficients appear: the two equations are wholly equivalent. In the general accepted frame of hypotheses when using the solution-diffusion model we have written Fick's equation when coupling between the fluxes exists. The order of magnitude of coupling is calculated, using previously published experimental data, leading to the conclusion that coupling, in selective pervaporation process, can be neglected.