Diffusive transport of a substance through a membrane separating solutions of different concentrations leads to the formation of concentration boundary layers (CBLs) adjacent to its surfaces. This phenomenon, commonly referred as concentration polarisation, causes the permeability coefficient measured in a membrane system to be smaller than the permeability of the membrane. The diffusion processes in the membrane system consisting of two cells separated by a horizontally located membrane were studied by using aqueous binary solutions of ethanol. To observe the time evolution of the CBLs, the laser interferometric technique was used: the interference fringe pattern provided quantitative measurement of the substance concentration C(x,t) at position x and time t. The concentration profiles were constructed by recording the interferograins, with a given time step Delta t; and on this basis, the CBL thicknesses were obtained. On the basis of these investigations, a practical and valuable method is proposed which can determine the membrane permeability coefficient under concentration polarisation conditions without the need for stirring solutions.