Factors controlling the δ13C of methane released by combustion include the combustion efficiency of the fire and the δ13C of the fuel. Smoldering fires produced 13C-depleted methane relative to hot, flaming fires in controlled forest and grassland burns and within a wood stove. Pine forest burns in the southeastern United States produced methane which ranged from −21 to −30‰, while African grassland burns varied from −17 to −26‰, depending upon combustion phase. African woodland burns produced methane at −30‰. In forest burns in the southeastern United States, the δ13C of methane released with smoldering was significantly 13C depleted relative to methane released under hot flaming conditions. Methane released with smoldering was depleted by 2–3‰ relative to the fuel δ13C, but this difference was not significant. The δ13C of methane produced in a variety of wood stove conditions varied from −9 to −25‰ and also depended upon combustion efficiency. Similar results were found for methane produced by gasoline automobile engines, where the δ13C of methane varied from −9 to −22‰. For combustion occurring within the confining chamber of a wood stove or engine the δ13C of methane was clearly 13C enriched relative to the δ13C of the fuel, possibly because of preferential combustion of 12CH4 in the gas phase. Significant quantities of ethylene (up to 25 to 50% of methane concentrations) were produced in southeastern U.S. forest fires, which may have consequences for physiological and reproductive responses of plants in the ecosystem. Methane production in these fires varied from 0.2 to 8.5% of the carbon dioxide production.