The microbiologically caused ennoblement appears in natural waters on all stainless steels equally and has been described in numerous publications. In addition to an external polarization of such a system, temperature, oxygen and pH-levels, supply of nutrients and/or the supply of substances to restrain biological activity have a direct influence on the rate of the potential rise. The final value of the potential is substantially regulated by the biological system and is independent of the steel composition. At the University of Applied Sciences in Konstanz, Germany, the potential rise of stainless steels has been examined in experimental set-ups. Up to recently, only effects on the open circuit potential of specimens have been measured. In order to further understand the mechanisms behind the potential rise. these specimens have been divided into micro-areas and Microelectrodes have been introduced to measure the electro-chemical potential of the micro-areas. The experimental set-up not only allows for the average mapping of the potential distribution over a specimen's surface, it further allows for learning more of the processes behind the rise of the electro-chemical potential. Influences of availability of nutrients, biocides, oxygen, and other factors on the free corrosion potential on the whole system as well as on the micro-areas have been examined and compared. It has been observed that the use of biocides resulted in a rapid decrease of the free corrosion potential, however only a very limited decrease of the potential within the micro-areas could be measured. Experiments with an external polarization of the specimens resulted in similar curves with quantitatively different potential differences. This paper is to present results of some of the conducted experiments with possible theories. (c) 2008 Elsevier Ltd. All rights reserved.