In recent years much effort has been devoted to assessing the influence of tobacco ingredients on the chemistry and toxicity of cigarette mainstream smoke. The techniques used to make the assessments are the pyrolysis of the ingredients and determination of the products of pyrolysis, especially those that are toxic, the influence of the ingredients on smoke constituents believed by regulatory authorities to be relevant to smoking-related diseases, influence of the ingredients on the in vitro genotoxicity and cytotoxicity of smoke particulate matter, and influence of the ingredients on the inhalation toxicity of smoke. Progress in the work undertaken by British American Tobacco has been presented at CORESTA meetings held in 2002, 2003 and 2004. All of the studies have indicated that commonly used tobacco ingredients do not change the toxicity of smoke as measured in specified assays. Also, the ingredients have no effect on the levels of most smoke constituents that may be relevant to smoking-related diseases. One possible exception to this generalisation is formaldehyde, which is generated from saccharide tobacco ingredients. In the present study, several individual saccharides commonly used as tobacco ingredients have been added to cigarettes, the cigarettes have been machine-smoked and the yields of formaldehyde in the resultant mainstream smoke have been compared to those from a control (no ingredient) cigarette. In addition, the formation of formaldehyde has been monitored during the pyrolysis of the saccharides. The results will be reviewed and their implications discussed.