Combustion in a constant volume chamber (CVCC) and an experimental direct injection (DI) engine was investigated with Fischer-Tropsch synthetic kerosene, GTL (gas-to-liquid fuel), in comparison to baseline ULSD#2. In CVCC the high reactivity fuel, GTL (62 CN) presented narrower NTC, indicating that as fuel reactivity increases, the region where NTC behavior takes place steadily shrinks. Stronger ignition reactivity and weaker NTC behavior produced a faster reaction rate compared with the ULSD case. Research was also conducted at 1300-1500 RPM and 40-70% load in the experimental medium duty diesel engine, operated in RCCI (Reactivity Controlled Compression Ignition) mode, obtained with early port fuel injection (PFI) of low reactivity fuel, n-butanol, and split DI of GTL at 60° BTDC and at TDC. Boost and PFI fraction (60% by mass) were kept constant in order to analyze the fuel reactivity effect on combustion phasing. The results showed that in order to maintain CA50 within 1 CAD, the mass injected of the higher reactivity GTL required for the first DI to decrease by 25% compared to ULSD#2 case. No LTHR and NTC were observed in the engine and decreased premixing diminished the maximum heat release rate for GTL given GTL’s high volatility and low viscosity. NOx decreased by 55% for GTL at 55-70% load resulting from lower combustion temperatures compared to ULSD#2 case. Using GTL, the soot levels were reduced across the load while maintaining RCCI combustion efficiency at 93-97%.