We studied the role of aerobic and anaerobic petroleum hydrocarbon degradation ata boreal, light-weight fuel and lubrication oil contaminated site undergoing naturalattenuation. At the site, anoxic conditions prevailed with high concentrations ofCH4 (up to 25% v/v) and CO2 (up to 18% v/v) in the soil gas throughout the year. Subsurface samples were obtained mainly from the anoxic parts of the site and they represented both the unsaturated and saturated zone. The samples wereincubated in microcosms at near in situ conditions (i.e. in situ temperature 8 °C, aerobic and anaerobic conditions, no nutrient amendments) resulting in the removal of mineral oil (as determined by gas chromatography) aerobically as well as anaerobically. In the aerobic microcosms on average 31% and 27% of the initial mineral oil was removed during a 3- and 4-month incubation, respectively. In the anaerobic microcosms, on average 44% and 15% of the initial mineral oil was removed during a 12- and 10-month anaerobic incubation, respectively, and e.g. n-alkanes from C11 to C15 were removed. A methane production rate of up to 2.5 μg CH4 h-1 g-1 dwt was recorded in thesemicrocosms. In the aerobic as well as anaerobic microcosms, typically 90% of themineral oil degraded belonged to the mineral oil fraction that eluted from the gaschromatograph after C10 and before C15, while 10% belonged to the fraction that eluted after C15 and before C40. Our results suggest that anaerobic petroleum hydrocarbon degradation, including n-alkane degradation, under methanogenic conditions plays a significant role in the natural attenuation in boreal conditions.