Reactivity controlled compression ignition (RCCI) is a form of dual-fuel combustion that exploits the reactivity difference between two fuels to control combustion phasing. This combustion approach limits the formation of oxides of nitrogen (NOX) and soot while retaining high thermal efficiency. The research presented herein was performed to determine the influences that high reactivity (diesel) fuel properties have on RCCI combustion characteristics, exhaust emissions, fuel efficiency, and the operable load range. A 4-cylinder, 1.9 liter, light-duty CI engine was converted to run on diesel fuel (high reactivity fuel) and compressed natural gas (CNG) (low reactivity fuel). The engine was operated at 2100 revolutions per minute (RPM), and at two different loads, 3.6 bar brake mean effective pressure (BMEP) and 6 bar BMEP. A matrix of nine different diesel fuels with varying cetane number (CN), aromatic content (AC), and distillation temperatures was used to identify high reactivity fuel property effects on RCCI combustion characteristics, exhaust emissions, fuel efficiency, and the operable load range. Results demonstrated that CN of the diesel fuel had a dominant effect on nearly all facets of RCCI operation. RCCI with fuels whose CN was lower than 33 resulted in higher NOX emissions and in-cylinder pressure rise rates (PRRs) compared to fuels with a CN ranging from 44 to 54. High CN fuels with a low AC (<23%) required the largest percentage CNG to maintain combustion phasing, 70.5% to 78.6% of total fuel energy input as CNG at 3.6 bar BMEP and 73.4% to 83.0% at 6 bar BMEP. High CN, low AC fuels also operated at the highest fuel conversion efficiency, 27.3% to 30.2% at 3.6 bar BMEP and 38.0% to 39.4% at 6 bar BMEP. In-cylinder PRR decreased as CN of the diesel fuel increased which would allow for higher loads to be achieved.