Emissions of fuel components from boating use on multiple-use lakes and reservoirs are of high concern with regard to the drinking water supply from such water bodies. We report results of a detailed study on the occurrence, sources and fate of arom. hydrocarbons and Me tert-Bu ether (MTBE) in a typical holomictic lake, Lake Zurich, that supplies drinking water for the largest Swiss city. Emphasis of the study was on the fuel oxygenate MTBE, which was found in concns. ?1.4 mg/L in the epilimnion and ?0.05 mg/L in the hypolimnion of the lake. The concn. difference was due to the stratification of the lake during the boating season with very limited water exchange across the thermocline. MTBE and BTEX nearly completely volatilized before vertical lake mixing occurred in winter. Spatial and temporal variations of MTBE concns. in the lake were obsd. and successfully predicted using 2 complementary box models (MASAS Light and Aquasim). The drinking water supply from holomictic lakes is not at risk for the scenarios studied if water is extd. from well below the thermocline. Since emissions of unburned gasoline into such water bodies are caused predominantly by boating activities, restrictions of highly emitting 2-stroke engines could substantially reduce the MTBE and BTEX load of the epilimnion during the boating season.