When the open system isotope method has been used to determine the methane oxidation efficiency of a landfill cover soil, it has been assumed that gas transport from the landfill is primarily driven by advection, a mechanism that is not associated with isotopic fractionation. A controlled laboratory experiment revealed that this approach underestimated the methane oxidation efficiency because it underestimated the importance of molecular diffusion during gas transport. In a worst-case scenario laboratory column experiment where diffusion was an important gas transport mode, a comparison between a mass balance and the open system isotope method revealed that the latter underestimated methane oxidation by a factor 2 to 4. The vertical profile of the δ13C value of methane in the column confirmed that isotope fractionation associated with gas transport occurred. Similar profiles were observed in the field, but the effect was less pronounced. It is concluded that the isotope method as currently applied produces a conservative estimate of methane oxidation by landfill cover soils.