This study presents the corrosion and microbiologically influenced corrosion processes of steels in the highly mineralized geothermal water of Polish Lowland. The geothermal sediments were studied using various techniques to confirm the presence of sulphate-reducing bacteria, to determine the chemical composition of deposits and to describe their morphology. On-site and laboratory electrochemical tests were carried out to determine the corrosion rate of selected steels and the influence of sulphate-reducing bacteria to the electrochemical processes.