This paper reviews the emissions of reactive nitrogen compounds (RNCs) from modern vehicles fitted with spark ignition en-gines and three-way catalysts. Specific aspects of the pollutants involved – and their formation – are discussed. Cold start driving cycles are scenarios under which emissions of all four RNCs can be significant; the mechanisms behind emissions trends are ex-plored. Experimental data obtained from two vehicles tested over two different cold start driving cycles are presented and analysed. The use of gravimetric and molar metrics are explored. Ammonia, a species which is currently not regulated for passenger cars in any automotive market, is identified as forming the majority of the RNC emissions over the entire driving cycle. While ammonia emissions are strongly linked to aftertreatment system warmup and periods of high load, significant ammonia emissions were also measured under certain hot-running, low load conditions, and even at idle. For the majority of the duration of the test procedures employed, the RNC profile was dominated by ammonia, which accounted for between 69% and 86% of measured RNCs in the ex-haust gas. Emissions are compared to the available legislative precedents (i.e. emissions limits currently in force in various jurisdic-tions). Finally, possibilities for control of exhaust emissions of currently unregulated RNCs are briefly discussed.