AbstractTransport sector decarbonisation is leading to increased demand for electrified powertrains including hybrid vehicles. The presence of an internal combustion engine and electric motor offer multiple performance and efficiency advantages. However, changes in the conditions that catalytic aftertreatment systems are subjected to can present challenges in meeting forthcoming emissions standards. This work investigated the three-way catalyst performance to abate regulated and unregulated emissions from a gasoline direct injection engine working under conditions related to hybrid vehicle operation. The focus on unregulated emissions of NH₃ and N₂O is of interest due to limited literature on their formation in conventional, and particularly hybrid, vehicle aftertreatment systems. Furthermore, the likelihood of their regulation when the EURO 7 emissions standards are introduced increases the pertinence of this work. For this particular engine and aftertreatment setup, it was found that starting the engine whilst the three-way catalyst temperature was below 150 ℃ led to an increase in tailpipe regulated emissions and N₂O. Whilst, starting the engine when three-way catalyst temperatures were above 350 ℃ lead to tailpipe NH₃ emissions. This was due to the selectivity of NO to form N₂O at lower temperatures and NH₃ at higher temperatures. For the case of the studied catalyst, a vehicle energy management strategy opting to start the engine with the three-way catalyst within a targeted temperature range allowed for a trade-off between regulated emissions, N₂O and NH₃. These findings are significant since it can be used to optimise hybrid vehicle control strategies minimising both regulated and unregulated emissions.