The focus of the present study was to characterize the fuel reactivity of high octane number fuels (i.e., low fuel reactivity), namely gasoline, ethanol, and methanol when mixed with cetane improvers under lean, premixed combustion conditions. Two commercially available cetane improvers, 2-ethylhexyl nitrate and di-tert-butyl peroxide, were used in the study. First, blends of the primary reference fuels iso-octane and n-heptane were port injected under fixed operating conditions. The resulting combustion phasings were used to generate effective PRF number maps. Then, blends of the aforementioned base fuels and cetane improvers were tested under the same lean premixed conditions as the PRF blends. Based on the combustion phasing results of the base fuel and cetane improver mixture, the effective PRF number, or octane number, could be determined. In all three base fuels it was found that 2-ethylhexyl nitrate is more effective at increasing fuel reactivity compared to di-tert-butyl peroxide. However, 2-ethylhexyl nitrate has a potential disadvantage due its nitrate group, which can manifest itself as NOx emissions. The relationship between the fuel-bound nitrate group and the engine-out NOx emissions was extensively characterized in the present study. It was also observed that methanol's response to cetane improvers was better than that of ethanol, in spite of the fact that they have similar octane numbers in their neat form. Once the reactivity of the base fuels was characterized, two mixtures of methanol and cetane improvers were selected and compared to diesel fuel as the high reactivity fuel (i.e., direct injected) for RCCI combustion.