We report experiments on conducting polymer polypyrrole leading to sensor applications. The resonance frequency, stability and sensitivity of AT-cut quartz crystals, electrochemically covered with polypyrrole tosylate: were tasted under various operating conditions, The interaction of this organic semiconductor with thin metallic films was analysed by photoelectron investigations. Shifts of the valence band spectra during silver deposition are explained by the neutralization of positive charged surface defects. Thus the existence of a space charge region in polypyrrole is demonstrated spectroscopically.