Variations in the isotopic composition (&dgr;13C,&dgr;D) of methane produced within a landfill site near Mainz, Germany, were studied using a newly developed tunable diode laser absorption spectrometer (TDLAS) method. Additional data on the mixing ratios of CO2, O2, N2, CH4itself and &dgr;13C of the CO2 in the landfill gas were also acquired. Samples taken from several branches of the landfill biogas collection system had methane isotopic compositions in the range &dgr;13C=-62.3 to -55.3% VPDB (n=23) and &dgr;D=-327 to -287% VSMOW (n=23). Although the variability of the stable isotope ratios is small, several significant correlations were found between these and the other measured parameters, which provide insight into the microbiological processes occurring within the landfill. Several samples showed evidence of admixture of atmospheric air which occurs when the pumping rate in the collection branch exceeds the local methane production rate. A fraction of the atmospheric oxygen is consumed during the passage through the landfill and CO2 is produced in addition to the CO2 associated with methanogenesis. The consumption of oxygen is correlated with the &dgr;13C and &dgr;D of CH4 and the &dgr;13C of CO2. The correlation is consistent with partial bacterial oxidation of CH4 resulting in the progressive enrichment of the remaining CH4 (&agr;(&dgr;13C)=1.008¿0.003 and &agr;(&dgr;D)=1.044¿0.020) and in the formation of very depleted CO2. For samples showing no evidence of oxidation, there was a negative correlation between &dgr;D and &dgr;13C(CH4) (r=-0.86,n=14) and between &dgr;13C(CO2) and &dgr;13C(CH4) (r=-0.95, n=14), which we interpret as originating from slightly varying contributions from the two methanogenic pathways CO2 reduction and acetate fermentation. ¿ American Geophysical Union 1995