Abstract: The number average molecular weight for polystyrene standards, poly-vinylchloride fraction and unfraction, vinyl chloride/vinyl acetate/vinyl alcohol copolymer, polymethylmethacrylate and polyacrylonitryle have been determined by a improved osmometer included one membrane and two chambers, and a simple dynamic osmometry. The osmotic pressure was determined by passing a flow of solvent through the membrane into the solution and extrapolating to zero flow rate. THF, acetone and DMFhave been used satisfactory as solvent in the measurement. The method is co-vennient to operate, rapid and reliable. Individual molecular weight measurements are accurete within ± 5 %.