Ammonia emissions from motor vehicles have risen significantly in recent decades, due to the rollout of three-way catalysts within the spark ignition vehicle fleet. Vehicular ammonia emissions are currently unregulated, even though ammonia is harmful for a variety of reasons, and the gas is classed as toxic. Having reached its light-off temperature, a three-way catalyst can produce substantial quantities of ammonia through various reaction pathways. Emission is markedly higher during periods where demand for engine power is higher, when the engine will be operating under open-loop conditions. The air-to-fuel ratio, cylinder temperature and rate of formation of carbon monoxide and nitrogen monoxide all correlate to varying degrees with ammonia production. Ammonia emissions could represent a serious threat to air quality, particularly in urban settings. Further investigation is imperative to minimise the risks posed by this as yet unregulated exhaust emission component.