Rapid compression machines can be used to measure a fuel’s ignition delay time and develop an understanding of its resistance to autoignition. Continuing developments in engine design demand higher octane fuels that are resistant to autoignition. Substituted phenols are members of the aromatic hydrocarbon family, and aromatics like toluene are often added to pump-grade gasoline to increase the fuel octane number. Previous numerical and experimental studies have found that substituted phenols included at additive levels in gasoline surrogates, such as the toluene reference fuel in this study, may have a lengthening effect on the ignition delay time of the base fuel they are added to. In this study, the substituted phenols 2,4-xylenol and 3,5-xylenol were added to their own respective toluene reference fuel at 2% by mole, and the ignition delay times of the base fuel mixtures including substituted phenols were compared to the base fuel without additives at engine-relevant conditions for equivalence ratios of φ = 0.6, 0.8, and 1.0. In combination with fuel composition, exhaust gas recirculation has been used to lengthen the ignition delay time of fuel-air mixtures, where exhaust gas recirculation helps to inhibit autoignition of fuel-air mixtures by lowering the combustion temperature of the mixture. The effect of exhaust gas recirculation on the ignition delay time of different fuel compositions was studied by using nitrogen as a diluent. Exhaust gas recirculation was carried out by adding N2 at rates of 15% and 30% for the stoichiometric fuel-air mixtures. Both additives were found to lengthen the ignition delay of the toluene reference fuel at high and low temperatures, but not at mid-range temperatures. 2,4-xylenol had a lengthening effect on the toluene reference fuel at more conditions than 3,5-xylenol did.