An optical refractometric technique has been used to acquire detailed spatially and temporally resolved concentration and concentration gradient measurements within an ultrafiltration concentration polarization layer. Solutions containing the polymer hyaluronan (M-n=434 000; M-w=605 000; M-z=827 000) in aqueous 10 mM NaCl and phosphate buffer solutions were added to a dead-end ultrafiltration flow cell and solvent was pumped through the layer at a constant flow rate. Membrane type (hydrophobic vs. hydrophilic track-etched polycarbonate), solvent type, initial polymer concentration, and pumping flow rate were changed systematically to quantify the effects of these parameters on the spatial and temporal development of the concentration polarization layer, and also on the pressure drop across the layer. The acquired data show discrepancies with theoretical predictions based on osmotic pressure and porous media modelling. These discrepancies may be due to a paucity of physico-chemical data required in the modelling, or the presence of mechanical stresses within the polarized layer not taken into account in the theory. An improved approach for fitting polarized layer concentration profiles is described.