The sedimentation equilibrium of sodium polystyrene sulfonate (NaPSS) in aqueous solution containing small amounts of simple salt was studied. Using an ionic dye having an absorption band separate from that of NaPSS as simple salt makes it possible to monitor the concentration gradients of both components simultaneously. The coupling of the two opposite gradients via a Donnan-type equilibrium allows the calculation of the osmotic coefficient of the NaPSS. The data thus obtained show a distinct decrease with increasing molar mass of the polyelectrolyte. A value of 0.22 as expected according to Manning＇s limiting law was found for molar masses up to 200 000 g.mol(-1). For higher molar masses, phi drops down to 0.1 at M-w = 10(6) g.mol(-1).