The hydrodynamics of flow structure in an industrial spiral reverse-osmosis device has been studied using as an example commercially available membrane elements ERO-E-6.5/900A and ERO-K-92-475 at operating pressures of 1.0 to 4.0 MPa. Experimental data on response curves and the axial mixing coefficient depending on the flow rate of the solution and pressure in the separation module have been obtained, thus making it possible to derive a criterion equation for calculating axial mixing coefficients. Numerical values for empirical factors are reported by which axial mixing coefficients can be calculated and their values be predicted for similar membrane units operating at a high hydrostatic pressure.