A solute flow model of the equations for a double-membrane system, in which a series of two (M-l and M-r) vertically mounted flat, microporous and symmetric polymer membranes separate three compartments (l, m, r) containing the non-homogeneous, non-ionic solutions was developed. Solution concentrations fulfilled the condition C-k(l) > C-k(m) > C-k(r). The intermembrane compartment (m) consists of an infinitesimally layer of solution. The volume of the 'compartments (l, m, r) fulfill the condition V-l = V-r approximate to 170V(m). The experimental tests were performed for binary (aqueous solutions of glucose or ethanol) or ternary (glucose solutions in 0.75 mol(.)l(-1) solution of ethanol or ethanol solutions in 0.1 mol(.)l(-1) aqueous solution of glucose) solutions. The linear dependencies of the solute flux on the concentration difference in binary solutions and non-linear - in ternary solutions - were obtained. It is shown that the double-membrane system has amplifying properties of solute flows. The results obtained during experiments were interpreted in the categories of convective instability, which increased the value of diffusive permeability coefficient of the system: concentration boundary layer/membrane/ concentration boundary layer.