Concrete exposed to sewage or industrial waste in the presence of air and inorg. reduced S compds. often degrades rapidly. S-oxidizing bacteria of the genus Thiobacillus that generate H2SO4 as the end product of their metab. play an important role in this process. To evaluate the resistance of concrete to the activity of these microorganisms, a specially designed H2S chamber contg. concrete test blocks was built. Temp. humidity, H2S concn., and exposure to aerosols of different thiobacilli are controlled in this chamber. Expts. show that the rate of concrete degrdn. is accelerated, and corrosion requiring at least 5 yr in sewer systems was reproducibly demonstrated within 9 mo. With this system, the degrdn. rates corresponding to wt. loss between 1 and 10% correlated most closely with densities between 106 and 108 cells/cm2 of the bacterium T. thiooxidans on the surface of the concrete test specimens. Specific polar lipid components in the membranes of the thiobacilli can be used to monitor the no. of these organisms on the surfaces of corroding concrete.