In order to characterize grafted polyelectrolyte layers based on electrokinetic measurements a theory of the surface conductivity K-sigma was developed, starting from the model of thick polyelectrolyte layers with uniform segment distribution and dissociable groups with an unknown pK value. According to this model the inner part of the polyelectrolyte layer adjacent to the substrate is considered to be isopotential while the potential decay occurs in a zone near the solution side of the layer. A simple equation for the Dorman potential Psi(D) as a function of pH, pK, electrolyte concentration C-0, and volume charge density p was obtained. In the derived equation K-sigma is directly related to Psi(D), while the other terms have less influence on the magnitude of K-sigma and can be accounted for in a second approximation using Psi(D) as determined from the measured K-sigma. Evaluation of the suggested model indicates that K-sigma measurements provide an effective method to characterize polyelectrolyte layers by analyzing the dependence Of Psi(D) on pH and CO: The magnitude of K-sigma yields information about the surface charge at complete dissociation of the ionizable groups. The dependence of K-sigma on pH and C-0 can be used for the determination of the pK value of the dissociating functions and the segment volume fraction of the polyelectrolyte can be estimated using the measured value of p. Published by Elsevier Inc.