The mean isotopic composition of CH4 emitted from six New England (United States) landfills was 13C and D enriched (-48.1 to -50.4? and -273 to -281?) relative to anoxic zone landfill CH4 (mean values of -55.9 to -56.2? and -296 to -300?) owing to the oxidation of methane as it was transported from the landfill to the atmosphere through the soil cap. The fraction of methane oxidized fO during its passage through the soil cap was calculated from the degree of 13C enrichment in emitted CH4 relative to anoxic zone CH4 in conjunction with values determined for the preference of soil methane oxidizing bacteria for 12CH4 over 13CH4 (α=1.022¿0.008). Mean values for methane oxidation in six landfills were from 24 to 35% of the total flux through the soil during the warm season, depending upon how the data were grouped. Our results bracket recent estimates of methane oxidation of about 30% in the warm summer period produced using a model with the input terms of soil temperature, moisture, depth, and oxygen concentration. Because of variations in the response of methane oxidation to temperature at these New England sites, our study is consistent with the modeling results of Czepiel et al. <1996b> that the best estimate for the annual value for methane oxidation in the landfills considered is about 10%. ¿ 1998 American Geophysical Union