The influence of O, pH, and substrate loading on microbially induced corrosion (MIC), compn. of microbial biofilm consortia, their activity, and corrosion products were detd. using a mini-plant, of which components were also described. Samples of mild and stainless steel were incubated with mixed cultures of Thiobacillus (T) intermedius, Desulfovibrio vulgaris, and Ochrobacter anthropi. The biol. and chem. activities of the biofilm samples were detd. by the heat output of microcalorimetry measurements. The oxidn. of Fe and reduced S compds. were responsible for the high chem. activity. By shortening the length of aeration, the content of S decreased, and, hence, the mass loss rate. The simultaneous presence of sulfate-reducing bacteria and T seemed to maintain a S cycle in the biofilm.