We have investigated the electrical transport properties of poly(3,4-ethylenedioxythiophen)/poly(4-styrene-sulfonate) (PEDOT:PSS) with PEDOT-to-PSS ratios from 1:1 to 1:30. By combining impedance spectroscopy with thermoelectric measurements, we are able to independently determine the variation of electrical conductivity and charge carrier density with PSS content. We find the charge carrier density to be independent of the PSS content. Using a generalized effective media theory, we show that the electrical conductivity in PEDOT:PSS can be understood as percolation between sites of highly conducting PEDOT:PSS complexes with a conductivity of 2.3 (Ocm)-1 in a matrix of excess PSS with a low conductivity of 10-3 (O cm)-1. In addition to the transport properties, the thermoelectric power factors and Seebeck coefficients have been determined. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012