Seasonal variations in the oxidation of methane during its transport across the soil cap of a landfill in Leon County, Florida, were determined in situ with a stable isotopic technique. The approach contrasted the &dgr;13C values of emitted and anoxic zone CH4 and utilized measurements of the isotopic fractionation factor &agr;, which varied inversely with temperature from 1.025 to 1.049. Anoxic zone CH4 did not vary seasonally and had a &dgr;13C average value of -55.18¿0.15?. Methane emitted from the landfill soil surface and captured in chambers ranged in &dgr;13C from -54? in winter, when emission rates were high, to -40? in summer, when emission rates were lower. The antipathetic variation between the &dgr;13C of emitted CH4 and the rate of CH4 emission is consistent with control of the emission rate by bacterial oxidation. Our interpretation of the isotope data indicates that methane oxidation consumed from 3 to 5% of the total flux in winter to a maximum of 43¿10% in summer. There was variation in the extent of methane oxidation in soil types, with mulch/topsoil averaging 55¿14% and clay averaging 33¿13% in summer. The seasonally integrated value for methane oxidation for areas of the landfill covered with mulch/topsoil was 26¿4% of the flux toward the soil surface, while for clay soil it was only 14¿2%. The overall annual average, which includes both types of soil, was 20¿3%. Covering land-fills with additional mulch, which can be generated from yard waste, may attenuate methane emission by providing a loose noncompact substrate for bacterial attachment and an environment with moisture, methane, and oxygen. At specific sites within the landfill we studied, temperature was the main factor controlling methane oxidation. ¿ 2000 American Geophysical Union