Under certain conditions, the uncontrolled migration of natural gas in the subsurface can result in an explosion, with the potential for significant property damage and loss of life. In western Pennsylvania, there are numerous possible sources of stray natural gas, including producing and abandoned wells, transportation pipelines, subsurface storage reservoirs, coal mines, and municipal waste landfills. The landfill gases are strictly of microbial origin, whereas the others are largely thermogenic in origin. Baseline geochemical data from gases generated in municipal landfills and those produced from gas wells and coals in western Pennsylvania help to identify sources of stray methane while investigating gas migration incidents. Landfill gases exhibit C1/(C2+C3) ratios >3.8X10 3 , delta 13 CO 2 values of -19.7 to + 17.4 per mil, delta DCH 4 values of -197 to -353 per mil, and delta 13 CH 4 values of -33.2 to -57.4 per mil. Most delta 13 CO 2 values are positive, whereas most methane, carbon, and hydrogen are isotopically light. Exceptions to these generalizations can be traced to isotope fractionation due to methane oxidation in soil environments. 14 C activity in the landfill methanes ranges from 125 to 145% modern. Thermogenic methane in the subsurface rocks of the region have delta 13 C and delta D values of approximately -27 to -55 per mil and -150 to -303 per mil, respectively. Chemical and isotopic analyses, combined with a thorough site investigation, provide a comprehensive approach for differentiating microbial from thermogenic gases and thermogenic gases from different sources. We used such an approach to confirm the offsite migration of landfill methane from a municipal landfill and the migration of stray methane from an abandoned gas pool to the soils adjacent to a commercial office building in western Pennsylvania.