The isotopic characteristics of municipal landfill leachate and gases (carbon dioxide and methane) are unique relative to the aqueous and gaseous media in most other natural geologic environments. The δ¹³C of the CO₂ in landfills is significantly enriched in ¹³C, with values as high as +20 ‰ reported. The δ¹³C and δD values of the methane fall within a range of values representative of microbial methane produced primarily by the acetate-fermentation process. The δD of landfill leachate is strongly enriched in deuterium, by approximately 30 ‰ to nearly 60 ‰ relative to local average precipitation values. This deuterium enrichment is undoubtedly due to the extensive production of microbial methane within the limited reservoir of a landfill. The concentration of the radiogenic isotopes, ¹⁴C and ³H, are significantly elevated in both landfill leachate and methane. The ¹⁴C values range between approximately 120 and 170 pMC and can be explained by the input of organic material that was affected by the increased ¹⁴C content of atmospheric CO2 caused by atmospheric testing of nuclear devices. The tritium measured in leachate, however, is often too high to be explained by previous atmospheric levels and must come from material buried within the landfill. The unique isotopic characteristics observed in landfill leachates and gases provide a very useful technique for confirming whether contamination is from a municipal landfill or some other local source.