Ammonia (NH3) is a promising fuel for use in marine applications due to its favorable chemical properties. However, emissions of especially nitrous oxide (N2O), with a global warming potential (GWP) of 273, pose a significant barrier to the adoption of NH3 as a climate-friendly carbon-free fuel. Removing N2O from the potential exhaust gas of NH3-engines is particularly challenging, not only because its activation requires high temperature, but also because of the inherent complexity of the exhaust gas mixture, including unburnt NH3 and nitrogen oxides (NOx), beside water vapor and oxygen. In this work, we studied relevant but anonymous samples from industrial partners under conditions of N2O decomposition and of simultaneous conversion of N2O/NOx/NH3. These catalysts displayed high-temperature N2O decomposition activity as well as inhibitory effects by oxygen and especially water. In the case of the simultaneous conversion of N2O and NOx, initial indications of the reaction pathways suggest the presence of different active sites for N2O decomposition and NOx reduction and suggest a plausible promotional role of N2O on NOx conversion coupled with a positive effect of the reducing agent NH3 on N2O conversion. However, considering a reaction environment with a water content potentially as high as ca. 30 vol%, preliminary hydrothermal aging tests revealed a significant negative impact on both N2O and NOx conversions that needs to be addressed in the development of practice-relevant catalyst.