From measurement of surface potentials of quartz capillaries before and after adsorption of PEO of various molecular masses, an assessment of the equilibrium hydrodynamic thickness of adsorbed layers delta has been obtained. When delta values are calculated with the Gouy equation, the psi(10) potentials of the quartz surface under the adsorbed layer of PEO are used. The zeta potentials measured at a very low concentration of the polymer solution, 10(-4) g/dm(3), were adopted as the phi(10), values. This method gives data that coincide better with the results of independent measurements of delta. The flow of polymer solution under increasing pressure differences causes deformation of adsorbed PEO layers, which manifests itself in changes in hydrodynamic thickness. The delta values decrease by 2-3 times when the tangential shear stress tau increases to 200 N/m(2). Such high values of tau were realized by using thin capillaries, 5-6 mu m in radius, and by applying the capillary electrokinetics method.