A new approach to the hydrocarbons (HC) speciation in autovehicular exhaust is presented here based on the addition of target deuterated HC to the fuel. Since deuterated compounds cannot be formed during the combustion process, their emission allow to assess their origin from the deuterated / hydrogenated concentration ratio in the fuel and in the exhaust. The percentage of unburnt and formed compound in the emission can be measured, discriminating the contribution of its original concentration from that of the remaining fuel components. Furthermore it is possible to verify the relationship between potential precursors and deuterated, or partly deuterated, metabolites. Similarly the HC present in the particulate emission can be traced. Monitoring the persistence of deuterated compounds in the exhaust, after their use, is a valuable tool for checking carry over effects.