Construction and use of a novel, semi-automated apparatus for concentrating aqueous polymer solutions by means of a reversibly thermosensitive hydrogel are described. The gel was chemically crosslinked poly(N-isopropylacrylamide-co-acrylic acid), which has a lower critical swelling temperature (T-c) of 37 degreesC, and the solute in the test solution was Dextran Blue (molecular weight 2 x 10(6) g mol(-1)). The hydrogel is placed in a double-walled cell, which is maintained at a temperature below or above T-c by passage of water from external baths. Switching between the two temperatures is effected by solenoid valves. Test solution is transferred from a reservoir into the cell, the how rate and hence mass of test solution being controlled by another solenoid valve. After swelling of hydrogel at the lower temperature the resultant, more concentrated test solution ('raffinate') is removed via a drainage stopcock and its concentration measured by VIS spectroscopy. The hydrogel is then deswollen by switching to the higher temperature mode, the water thereby expelled being removed via the drainage stopcock. A considerable level of automation of the apparatus is achieved by use of individual timers for each of the five solenoid valves. Separation factors and efficiencies of separation were evaluated and found to be dependent on (a) time of swelling; in some cases times were deliberately kept short and final equilibrium swelling was not reached, (b) concentration of test solution and (c) mass of test solution relative to that of the initial gel.