[en] Natural bitumen can be an aid in the assessment of the long-term behaviour of technical bitumen. Natural occurrences of bitumen usually have the drawback that the original material before alteration or degration began is not known. The present work applies an alternative approach: on the basis of the geology and stratigraphy at a site where bitumen samples have been taken, the existence of a gradient in the parameters of subaerial and microbial degradation processes may be assumed. Therefore relative variation in properties, composition and structure (bitumen content, volative fraction, elemental composition, chromatographic fractions, saturated hydrocarbons, trace metals, humic substances) at different locations within the deposit have been analysed. The bitumen impregnates a bed of porous Jurassic limestone which crops out at the surface and dips under sediments of various permeability. The quality of the bitumen is in compliance with standards for soft technical bitumen, although it can be characterized as highly biodegraded. It has probably not been affected to any marked degree by degradation since the Quarternary or possibly even late Tertiary, however, as observable variations in composition and properties are only minor and seem to exhibit no clear relation to the present geological setting. Only near the present outcrop do there seem to be signs of increased oxidation. Direct exposure to the weather at the surface leads to destruction of the bitumen within a very thin surface layer. Traces of humic substances probably originating from the decomposition of bitumen were found. The rate of bitumen degradation at outcrops seems to coincide with the rate of weathering and erosion of the host rock. It can be concluded from the results that the time scales necessary to achieve significant alteration of bitumen under the given conditions far exceed the time scales dictated by the half-lives of radionuclides in low- and medium-level radioactive waste