Formation profiles have been obtained for methane, ethane, ethene, propane, propene, butanes, butenes, isoprene, formaldehyde, acetaldehyde, acetone, 2-butanone, benzene, and toluene from the thermal decomposition of tobacco in the presence of helium and air. These data show that in helium the temperatures for optimum formation of gas phase constituents were: hydrocarbons, 450°C; aldehydes, 300°C; ketones, 450°C; isoprene, 380° and 475°C; and aromatic hydrocarbons, 450°C. Air enhances the formation of these gas phase constituents at 280°C and in most cases at 420°C, the latter temperature is an area of major weight loss of tobacco. Each formation maximum corresponds to a rate of weight loss maximum exhibited by derivative thermogravimetry. The results also show that it is possible to use effluent gas analysis to define the thermal behaviour of tobacco in terms of the formation of the gas phase constituents which provide a means to elicit the processes that occur during the thermal decomposition of tobacco.