Concentration polarization phenomenon in ultrafiltration or reverse osmosis processes is governed by a non linear convective-diffusion equation. General unsteady state equations associated with the osmotic theory relationship are solved numerically. Unstirred ultrafiltration flux declines are accurately described when the concentration dependence of the osmotic pressure or diffusion coefficients are known. Inversely, when the experimental evolution of the permeate flux is introduced in the calculation, it is possible to determine osmotic pressure or diffusion coefficients as a function of solute concentration with a good precision. The results obtained for four different kinds of well-defined non charged polymers agreed well with available literature data.