In this paper we report on the development of an automated, laser-based refractometric technique for the measurement of concentration and concentration gradient in the concentration polarization layer during dead-end ultrafiltration. Spatially and temporally resolved measurements can be made to within 200 mu m of the membrane. Solvent is pumped through the layer at a constant flow rate, and the driving pressure is also measured. The system was calibrated and tested using the biopolymer hyaluronan (M-n=434000; M-w=605000; M-z=827000). Good repeatability, accuracy, and sensitivity were demonstrated. These studies lay the foundation for a physico-chemical study of concentration polarization dynamics, reported in a companion paper.