A review is given. The fate of fuel oxygenates such as Me tert-Bu ether (MTBE) in the subsurface is governed by their degradability under various redox conditions. The key intermediate in degrdn. of MTBE and Et tert-Bu ether (ETBE) is tert-Bu alc. (TBA) which was often found as accumulating intermediate or dead-end product in lab. studies using microcosms or isolated cell suspensions. This paper discusses the thermodn. of the degrdn. processes utilizing various terminal electron acceptors, and the aerobic degrdn. pathways of MTBE and TBA. It summarizes the present knowledge on MTBE and TBA degrdn. gained from either microcosm or pure culture studies and emphasizes the potential of compd.-specific isotope anal. (CSIA) for identification and quantification of degrdn. processes of slowly biodegradable pollutants such as MTBE and TBA. Microcosm studies demonstrated that MTBE and TBA may be biodegradable under oxic and nearly all anoxic conditions, although results of various studies are often contradictory, which suggests that site-specific conditions are important parameters. TBA degrdn. has not been shown under methanogenic conditions and it is currently widely accepted that TBA is a recalcitrant dead-end product of MTBE under these conditions. Reliable in situ degrdn. rates for MTBE and TBA under various geochem. conditions are not yet available. Degrdn. pathways under anoxic conditions have not yet been elucidated. All pure cultures capable of MTBE or TBA degrdn. isolated so far use O as terminal electron acceptor. In general, compared with hydrocarbons present in gasoline, fuel oxygenates biodegrade much slower, if at all. The presence of MTBE and related compds. in groundwater therefore frequently limits the use of in situ biodegrdn. as remediation option at gasoline-contaminated sites. Though degrdn. of MTBE and TBA in field studies has been reported under oxic conditions, there is hardly any evidence of substantial degrdn. in the absence of O. The increasing availability of field data from CSIA will foster our understanding and may even allow the quantification of degrdn. of these recalcitrant compds. Such information will help to elucidate the crucial factors of site-specific biogeochem. conditions that govern the capability of intrinsic oxygenate degrdn.