We report the high temperature thermoelectric properties of solution processed untreated and sulphuric acid treated poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (or PEDOT:PSS) films. The acid treatment is shown to simultaneously enhance the electrical conductivity and the Seebeck coefficient of the metal-like films, resulting in a five-fold increase in the thermoelectric power factor (from 0.01 to 0.052 W m-1 K-1) at 460 K, compared to the untreated film. By using atomic force micrographs, Raman and impedance spectra and using a series heterogeneous model for electrical conductivity, we demonstrate that acid treatment results in the removal of PSS from the films, leading to the quenching of accumulated charge-induced energy barriers, facilitating metal-like conduction. The continuous removal of PSS and changes in morphology of the PEDOT grains upon acid treatment may alter the local band structure of PEDOT:PSS, in such a way as to simultaneously enhance the Seebeck coefficient. © 2015 The Royal Society of Chemistry.