Numerous research activities are conducted all over the world to study biological treatment of H2S in laboratory-scale bioreactors. Important hazards associated with these bioreactor systems include the escape of H2S gas and leakage of chemical/biological liquids, which have severe adverse effects on the involved labors, equipment, and materials. The objective of this article is to present a quantitative safety analysis of a laboratory-scale continuous bioreactor system for H2S gas biotreatment using the fault tree analysis approach. Three unwanted top events were determined as the most hazardous events, being H2S leakage inside the laboratory, H2S leakage to outdoor from bioreactor outlet, and leakage of liquid chemical/biological solutions. The minimal cut sets and the probability of the occurrence of each top event were determined. The importance of cut sets and basic events were calculated, and priorities for control measures were determined. The analysis allows better decision on priority of control measures, and maintenance or replacement schemes of the system components in an endeavor to minimize the probability of failure or hazard occurrence. The presented analysis proves the usefulness of fault tree analysis in making quantitative risk assessment and safety analysis, which are important elements in laboratory safety management system. (c) 2013 American Institute of Chemical Engineers Process Saf Prog 32: 376-386, 2013