Data for the transport of different series of solutes (poryethylene glycol, dextrans and proteins) are measured through asymmetric sulfonated polysulfone ultrafiltration membranes of various molecular weight cut-off and a polyvjnyldifluoride microfiltration membrane of nominal pore size 0.1 mu m. When sieving coefficients are plotted as a function of molecular weight, a curve is obtained for each series of solute while when data are plotted versus the hydrodynamic volume, sieving coefficients fall close to a single curve for each membrane. The hydrodynamic volume is therefore more appropriate than the molecular weight for a standard characterisation of membranes. Results are in good agreement with available hydrodynamics models for the hindrance factor of rigid spherical solutes through cylindrical pores, with the effective solute size evaluated from viscosimetric measurements and assuming a log-normal pore size distribution for the membrane.