In the process to reach the membrane-equilibrium in the aqueous solution of poly(vinylpyrrolidone)(PVP) and sodium dodecyl sulfate(SDS), it was found that osmosis of water occurred through the membrane as well as diffusion of SDS. By the osmosis through the membrane, water penetrated into the inner solution which contained PVP and the volume of the inner solution increased at the membrane equilibrium regardless of the initial condition of the ex. - periments. It was concluded that the penetration of water into the PVP-containing side oc- curred by the additional osmotic pressure which was due to the sodium ion dissociated from the SDS-PVP complex formed. This effect on the osmosis was small for aqueous PVP solutions without addition, or for the PVP solutions containing CaCl2 or dodecyl amine hydrochloride (DAH) which did not interact with PVP. On the other hand, the aqueous solutions of polyelectrolyte, such as sodium carboxymethyl cellulose(CMC-Na), showed this effect more markedly than the above mentioned systems, even without any added salt. It was, therefore, concluded that counter-ions dissociated from the polyelectrolytes or from the complex formed contributed to this osmotic phenomenon.Tryptophan(Tay.) formed complex with PVP, but amount of bound Try. was smaller than that of SDS, and the free concentration in equilibrium was larger than that of SDS, so the intensity of osmosis was small. This phenomenon was similar to the fact that when NaCI was added to aqueous solution of CMC-Na, intensity of osmosis decreased. It was, therefore, concluded that the osmotic pressure due to counter-ions from the polymer decreased and, accordingly, the osmotic phenomenon became less pronounced, when the concentration of added salt in polyelectrolyte solution or that of the free solute in complex-forming solution increased.