Cigarettes have been smoked under continuous draw conditions in an atmosphere containing oxygen-t8, in order to locate the regions where carbon monoxide and dioxide are formed by thermal decomposition and combustion of tobacco. The carbon oxides found inside the burning zone (combustion coal) at temperatures above 500ยฐC are formed largely by combustion of tobacco. Those found behind the coal at temperatures below 500ยฐC are produced mainly by thermal decomposition of tobacco. The gas concentration at a given position inside the cigarette is dependent on the net chemical production of the gas, and the net rates of diffusive and convective flow through that region. Similarly, the temperature at any point inside the cigarette is determined by the net heat released by local chemical reaction, as well as heat transfer by conduction, convection, and radiation. The net rates of formation of heat and gases by chemical reaction at various positions inside the cigarette have been calculated from the empirical temperature and gas distributions, using heat and mass transfer equations. There is a strong exothermic region inside the coal where oxygen is consumed and the carbon oxides are formed by combustion. There is a strong endothermic region behind the coal where the carbon oxides are formed by thermal decomposition of tobacco.