Hydrogen internal combustion engines (H2ICE) have emerged as a promising solution for decarbonisation of the transport sector, due to low cost and potential for rapid deployment. However, abnormal combustion and high nitrogen oxide (NOx) emissions limit stoichiometric operation, making dilution strategies essential. While lean combustion has been widely studied, combined dilution strategies of air and exhaust gas recirculation (EGR) require further investigation. This work presents experimental results from a boosted 0.5-litre spark-ignition direct-injection single-cylinder research engine equipped with high-tumble ports and cooled high-pressure EGR. Relative air–fuel ratios (lambda) of 1 to 3 and EGR rates of 0 to 40% are evaluated at 5, 10, and 15 bar of indicated mean effective pressure (IMEP) at 2000 rpm to assess effects on net indicated thermal efficiency (nITE), combustion, and emissions. A peak nITE of 43.5% is achieved at 10 bar IMEP, λ = 2.5, and 30% EGR, which can be primarily attributed to low heat losses while maintaining lower combustion losses than at higher dilution levels. NOx emissions are effectively mitigated with increasing EGR and are largely independent of lambda at 5 bar IMEP under EGR dilution. At high load, EGR is shown to be beneficial to achieve high efficiency and lower NOx at lower dilution rates, thereby reducing boosting requirements. Equivalent dilution parameters are used to investigate combined effects of EGR and air dilution, from a mass dilution perspective with the mass dilution rate (MDR) and equivalent thermal reduction with the thermal dilution parameter (TDP). Indicated efficiency and unburned hydrogen emissions correlated strongly with MDR, while temperature-dependent parameters showed a high correlation with TDP. At constant engine speed, burn durations are shown to depend mainly on degree of thermal dilution, with no effect of load observed. At high dilution rates, combustion became increasingly insensitive to further dilution, indicating the presence of thermodiffusive instabilities under high levels of both EGR and air dilution.