The work presents corrosion mechanism of iron alloys in H2O-CO2-H2S system in the respect of their corrosion resistance in geothermal waters. Anodic behaviour of carbon steel, ductile iron, low-alloyed steel and stainless steel in thermal water of Geotermia Podhalanska S.A. (Tatra Mountains) has been discussed. There has been also presented the influence Of CO2 and H2S on the stability of passive film in H2O-CO2-H2S system at elevated temperature and under high pressure. (c) 2007 Published by Elsevier Ltd.