Streaming potentials for glass capillaries with and without adsorbed poly(vinyl pyrrolidone) were used to determine the thickness of the adsorbed polymer layer. It was found that the thickness determined in this way is a strong function of the ionic strength of the solution. The results are compared with a recent theory and satisfactory agreement is found. The thickness obtained in the limit of very low ionic strength is identified with the hydrodynamic thickness, and the dependence of this hydrodynamic thickness on the adsorbed mass of polymer is studied. The results are strongly indicative of the occurrence of long tails sticking out of the adsorbed layer.