A proposal for a tracer method to be used in the investigation of fuel effects is made. In addition, preliminary results of a tracer investigation of exhaust emissions in a Wankel engine which burns an oil-fuel mixture (NSU KKM 507/3) are reported. It is hoped that the tracer technique proposed here will be able to elucidate the nature of the chemistry of the combustion process in engines, in particular those processes leading to atmospheric pollution. The initial study examined the relative contribution of each component of an oil-fuel mixture to incompletely burned material in the exhaust. It was found that the oil contributed a disproportionate amount to the total emissions prior to an engine overhaul when the rotor seals were replaced. Subsequent to this the proportion of incompletely burned material attributed to the oil was the same as the proportion of oil in the initial oil-fuel mixture.