Several membranes have been tested in a test loop especially built in order to achieve well-defined flow conditions at the membrane surface. A test solution of dextran was ultrafiltrated at 0.5 MPa and 25°C at five circulation velocities from 5 m/s to 1 m/s. This corresponds to Reynolds numbers of 20000--4000 at a channel height of 2 mm. For the evaluation of the results, the film-theory model, the three-parameter model of Kedem and Katchalsky and the osmotic pressure model were applied. The relationships between observed retention and circulation velocity as well as observed retention versus molecular weight were also considered. SYMBOLS C --concentration (kmol/m 3) --(C m --Cp)/ln (Cm/Cp) C b --solute concentration in the bulk (kmol/m 3 ) Cm --solute concentration at the membrane surface (kmol/m 3 ) Cp --solute concentration in the permeate (kmol/m 3) D --diffusion coefficient (m2/s) Dh --hydraulic diameter (m) Js --solute flux (kmol/m 2" s) Jv --volume flux (m3/m 2" s) h --mass transfer coefficient (m/s) Lp --pure water permeability (m3/m 2" S" MPa) Mn --number average mole weight Mw --weight average mole weight P --solute permeability (m/s) AP --pressure (Mea)