Methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) are mainly used as additives to gasoline in order to enhance the combustion efficiency. As an assignment from the Swedish Environmental Protection Agency, IVL has performed a screening study of MTBE and ETBE. The overall objective of the screening was to determine concentrations in a variety of media in the Swedish environment, in order to judge if the substances are commonly occurring in the Swedish environment. A further aim was to determine key sources and highlight major transport routes