Studies have been carried out of the thermal decomposition and combustion of cigarette paper with heating rates and final temperatures similar to those encountered during the smoking of cigarettes. Measurements have been made in particular of the amounts of carbon monoxide formed from the paper, both on its own and when it forms part of a cigarette. The extent of formation of char from cigarette paper decreases as the heating rate is increased, but carbon monoxide yields pass through a maximum at a heating rate of ca 60° s−1, due to the competing effects of temperature on the several different mechanisms by which carbon monoxide is produced. Measurements on cigarette papers impregnated with a wide range of inorganic and organic salts show that all the additives used tend to decrease carbon monoxide yields at low heating rates but to increase them quite sharply at very high ones. With cigarettes containing impregnated papers the concentration of carbon monoxide is increased in both the mainstream and sidestream smoke. Possible reasons for the behaviour observed with the various individual salts are discussed. Certainly, on the assumption that the results obtained with cigarettes are a good reflection of those on paper alone it seems unlikely that treatment of cigarette paper can ever reduce the amounts of carbon monoxide formed under cigarette smoking conditions, for all additives affect char formation in such a way that there is an overall increase in carbon monoxide yields.