Until recently it was believed that the occurrence of nitroso compounds in drinking water and wastewater was due to contamination of the source water. Recent observations indicate that N-nitrosodimethylamine (NDMA), a particularly potent carcinogen, can be produced during water and wastewater treatment. Studies at Iowa have recently elucidated a novel formation pathway that involves reactions of chlorine, ammonia, and dimethylamine that support the hypothesis that NDMA is a new disinfectant by-product (DBP). We propose that NDMA is but one representative of a new class of disinfectant by-products, the nitroso compounds, many of which are of health concern. We hypothesize that other types of nitroso compounds are also formed by a similar mechanism in chlorinated and chloraminated drinking and waster. This project will investigate the formation mechanism and occurrence of several nitroso compounds (not NDMA). This information is important in making comprehensive risk assessments of disinfection practices.