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Methane oxidation in landfill covers was determined by stable isotope analyses over 37 seasonal sampling events at 20 landfills with intermediate covers over four years. Values were calculated two ways: by assuming no isotopic fractionation during gas transport, which produces a conservative or minimum estimate, and by assuming limited isotopic fractionation with gas transport producing a higher estimate. Thus bracketed, the best assessment of mean oxidation within the soil covers from chamber captured emitted CH4 was 37.5 +/- 3.5%. The fraction of CH4 oxidized refers to the fraction of CH4 delivered to the base of the cover that was oxidized to CO2 and partitioned to microbial biomass instead of being emitted to the atmosphere as CH4 expressed as a percentage. Air samples were also collected at the surface of the landfill, and represent CH4 from soil, from leaking infrastructure, and from cover defects. A similar assessment of this data set yields 36.1 +/- 7.2% oxidation. Landfills in five climate types were investigated. The fraction oxidized in arid sites was significantly greater than oxidation in mediterranean sites, or cool and warm continental sites. Sub tropical sites had significantly lower CH4 oxidation than the other types of sites. This relationship may be explained by the observed inverse relationship between cover loading and fractional CH4 oxidation.