Reactivity controlled compression ignition combustion was investigated for three fuel combinations: isooctane-diesel, PRF90-diesel, and E85-diesel. Experiments were conducted at 1200 rpm, 160 kPa absolute intake pressure, and fixed total fuel energy using ‘optimal’ operating condition for each fuel combination that were chosen based on combustion performance from SOI timing and premixed energy fraction sweeps. The heat release duration was found to scale with the difference in reactivity between the premixed and direction injected fuel; a small difference gives rise to short heat release duration, similar to that of HCCI combustion. Conversely, as the difference increases, the heat release period lengthens. The high-speed optical data confirmed that the combustion occurred in a staged manner from the high-reactivity zones, which were located at the periphery of the chamber, to low-reactivity zones in the field of view. Further, the range of ignition timing across the imaged area was found to scale with the difference in the two fuel's reactivity. In cases where there was a small difference in reactivity, the ignition took place in a relatively narrow window within the heat release profile. In cases with large reactivity difference, the progression of the ignition front from high- to low-reactivity regions was found to occur over an extended part of the heat release duration. Despite the contrast in combustion characteristics and injection timings, similar integrated luminosity levels in the 310 nm range were observed for the tested fuels. The luminosity levels were comparable to HCCI combustion levels and in excess of two orders of magnitude lower than standard diesel combustion.