Intrinsic biodegradation, representing the key process in Natural Attenuation, was examined at a tar-oil polluted disposal site. Methods to assess microbial natural attenuation of BTEX and PAH included analysis of groundwater hydrochemistry, pollutant profiles, composition of the microflora, and microcosm studies. In the polluted groundwater downgradient the disposal site, oxygen and nitrate were only available adjacent to the groundwater table and at the plume fringes. In the anaerobic core of the plume, a sequence of predominating redox zones (methanogenic, sulphate-reducing, Fe(III)-reducing) was observed. Changing pollutant profiles in the plume indicated active biodegradation processes, e.g. biodegradation of toluene and naphthalene in the anaerobic zones. High numbers of microorganisms capable of growing under anaerobic conditions and of aerobic pollutant degrading organisms confirmed the impact of biodegradation at this site. In microcosm studies, the autochthonous microflora utilised toluene, ethylbenzene, and naphthalene under sulfate- and Fe(III)-reducing conditions. Additionally, benzene and phenanthrene were degraded in the presence of Fe(III). Under aerobic conditions, all BTEX and PAH were rapidly degraded. The microcosm studies in particular were suitable to examine the role of specific electron acceptors, and represented an important component of the multiple line of evidence concept to assess natural attenuation.