Thin films of an electrically conducting polymer blend, poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate), PEDOT-PSS, have been studied using X-ray and ultraviolet photoelectron spectroscopy. Special attention has been paid to thermal effects and the influence of hydrochloric acid on the chemical and electronic structures of the films, since these issues are important in applications of PEDOT-PSS as a hole-injecting electrode in polymer-based light emitting devices. Three chemically different species were identified in pristine PEDOT-PSS, namely poly(4-styrenesulfonic acid), poly(sodium 4-styrenesulfonate and poly(3,4-ethylenedioxythiophene). The HCl-treatment is shown to lead to the conversion of the sodium salt into the free poly(styrenesulfonic acid). Heating of PEDOT-PSS films resulted in a decomposition of the free sulfonic acid component (presumably through loss of SO3), whereas the sodium salt is thermally stable. It is shown that phase segregation occurs in the PEDOT-PSS system resulting in a surface entirely covered by PSS part of the PEDOT-PSS, as indicated by both XPS and UPS measurements.