The integration of thin conducting polymer films as an on-chip gas sensitive medium offers the possibility of new applications in microsystems technology. Due to the material properties of polythiophene (PTh) such as adjustable conducting capability, chemical stability and mechanical properties, this heterocyclic polymer attracts attention as a promising candidate for gas sensitive layers. PTh is considered to be stable in air and moisture especially in the undoped state. However, when considering the gas adsorption behaviour of PTh films in ambient conditions the O-3 load cannot be neglected as additional chemical states are created. These changes in the surface chemistry of PTh as a result of the O-3 load were investigated by X-ray photoelectron spectroscopy (XPS). The carbon C 1s and sulphur S 2p spectra show an increasing amount of oxidised species depending on the O-3 concentration used in the experiments. The chemical shifts of these peaks suggest the occurrence of carboxyl and SO2 groups for all O-3-loaded samples. The relative intensity of these peaks reaches saturated values at samples loaded between 200 and 600 ppb for 1 h. In order to find a correlation to the electrical conductivity versus various ozone loads, carbon shake-up states and angle resolved valence bands were evaluated and considered in one context.