The release of methane (CH4) from landfills to the atmosphere and the oxidation of CH( in the cover soils were quantified with static chambers and a C-13-isotope technique on two landfills in Sweden. One of the landfills had been closed and covered 17 years before this investigation while the other was recently covered. On bath landfills, the tops of the landfills were compared with the sloping parts in the summer and winter. Emitted CH4, captured in chambers, was significantly enriched in C-13 during summer compared with winter (P < 0.0001), and was enriched relative to anaerobic-zone methane, The difference between emitted and anaerobic zone delta C-13-CH4 was used to estimate soil methane oxidation. In summer, these differences ranged from 9 to 26 parts per thousand, and CA(4) oxidation was estimated to be between 41 and 50% of the produced CHI in the new landfill, and between 60 and 94% in the old landfill. In winter, when soil temperature was below 0 degreesC, no difference in delta C-13 was observed between emitted and anaerobic-zone CH4 suggesting that there was no soil oxidation. The temperature effect shown in this experiment suggests that there may be both seasonal and latitudinal differences in the importance of landfill CH4 oxidation. Finally the isotopic fractionation factor to) varied from 1.023 to 1.038 and was temperature dependent, increasing at colder temperatures. Methanotrophic bacteria appeared to have high growth efficiencies and the majority of the methane consumed in incubations did not result in immediate CO2 production.