One process determining the fate of xenobiotics (organics not natural to a system) in environmental waters is bacterial transformation. Bacteria are one of the most metabolically active groups of organisms in the environment. They can degrade and derive energy from a variety of organic compounds. Increases in the number of synthetic and their accumulation in the environment have led to a more quantitative and critical view of microbial metabolism of xenobiotics. Rates of microbial degradation depend upon both the structure of the compound and on the metabolic activity of the bacteria in the ecosystem receiving the xenobiotic. Studies of second-order kinetics are a useful beginning in the detailed quantitative study of rates of biotransformation of xenobiotics. Many questions about the role of temperatures of biotransformation of xenobiotics. Many questions about the role of temperature, other nutrients, population structure, and compound structure are still unanswered. Only additional kinetic studies for many compounds and sites can provide confidence in the approach and the ultimate answers to these questions.