Numerous studies have demonstrated that EGR can attenuate knock propensity in SI engines at naturally aspirated or lightly boosted conditions. In this study, we investigate the role of EGR under higher load conditions with multiple fuel compositions, where highly retarded combustion phasing typical of modern SI engines was used. It was found that under these conditions, EGR has little effect on mitigating the knock propensity and doesn’t allow significant combustion phasing advance as it does under lighter load conditions. Detailed combustion analysis shows that when EGR is added, the polytrophic coefficient increases causing the compressive pressure and temperature to increase. At sufficiently high boosting conditions, the increase in polytrophic coefficient and additional trapped mass from EGR can increase the pressure and temperature sufficiently to cause pre-spark heat release with some fuels. Further, chemical kinetic modeling demonstrates that the effectiveness EGR to mitigate knock is highly dependent on the pressure-temperature condition. Under highly boosted conditions relevant to modern downsized boosted SI engines, knock mitigation through EGR is reduced as charge reactivity increases at these conditions.