Still today, the final destiny of a large part of the waste dumped is landfilling, which is an important source of methane. Quantifying the amount of methane that is oxidized when the landfill gas passes through the seal, it can provide valuable information when determining the total GHG potential emission of a landfill. In this work, methane oxidation has been determined for landfill gas samples taken on the surface of three Spanish landfills. For this, it has been determined the 13C isotopic signal in methane by cavity ring-down spectroscopy (CRDS-WS). Previously, the preference of methanotrophic bacteria for 12C isotope versus 13C has been quantified using soil samples from the three landfills. The results obtained show a great variability, ranging from 0 to 70% of methane oxidized. This variability can be explained with the specific characteristics of each sample, suggesting a consistent methodology
