The prototype of a fast and versatile membrane osmometer based on a capillary shaped porous glass membrane has been developed and successfully tested with solutions of polystyrene in toluene. The coaxial design of the cell is ideally suited both for batch and continuous flow operation. Time constants on the order of several seconds are observed with membranes of 10 nm and 20 nm nominal pore size. Important design criteria are discussed and first results for the cut-off molar mass are reported.