This study essentially deals with the description and modeling of transport phenomena in ultrafiltration (UF). In the light of new and published experimental results obtained with solutions of dextran T500, the realism and validity of the most classical models such as the gel-polarization model, the osmotic pressure model, the resistance-in-series model have been first discussed. Experimental results are not in agreement with the first two models. Moreover, despite several modifications, the resistance-in-series model does not predict the permeate flux in a general way. The common failure of these simple models is to average all parameters (permeate flux. solute concentration) over the whole membrane length. As a result, it is impossible to use them to predict the influence of the membrane length. We have then developed a new two-dimensional model based on the numerical resolution of the convective and diffusion equations. Permeate fluxes predicted are in good agreement with experimental results and the respective influence of the velocity, the initial concentration, the transmembrane pressure and the membrane length are very well described.