Results from a large set of HCCI experiments performed on a single-cylinder research engine fueled with different mixtures of iso-octane and n-heptane are presented and discussed in this paper. The experiments are designed to scrutinize fuel reactivity effects on the operating range of an HCCI engine. The fuel effects on upper and lower operating limits are measured respectively by the maximum pressure rise rate inside the cylinder and the stability of engine operation as determined by cycle-to-cycle variations in IMEP. Another set of experiments that examine the intake air heating effects on HCCI engine performance, exhaust emissions and operating envelopes is also presented. The effects of fuel reactivity and intake air heating on the HCCI ranges are demonstrated by constructing the operating envelopes for the different test fuels and intake temperatures. The paper discusses, in the light of the results, how the nonlinearity in fuel effects makes the dual fuel control approach less effective in extending the lower end of the HCCI load range. It also discusses how intake air heating affects the engine operation stability at low loads, and how varying fuel reactivity and intake heating can complement each other as an integrated control approach to extend both ends of the HCCI load range.