Dual fuel reactivity controlled compression ignition (RCCI) combustion is a promising method to achieve high efficiency with near zero NOx and soot emissions; however, the requirement to carry two fuels on-board limits practical application. Advancements in catalytic reforming have demonstrated the ability to generate syngas (a mixture of CO and hydrogen) from a single hydrocarbon stream. This syngas mixture can then be used as the low reactivity fuel stream to enable single fuel RCCI combustion. The present effort uses a combination of engine experiments and system level modeling to investigate reformed fuel RCCI combustion. The impact of reformer composition is investigated by varying the syngas composition from 10% H2 to approximately 80% H2. A system level and second law analysis are performed on the highest efficiency operating points and comparisons are made between partial oxidation reforming and steam reforming. The results show that endothermic reforming (steam reforming) can improve system level efficiency compared to conventional diesel operation by recovering exhaust energy for similar system-out emissions characteristics. An auto-ignition integral approach combined with reformer equilibrium modelling shows that in order to achieve similar ignition characteristics of the background fuel for the range of hydrogen fractions tested, sub-optimal reforming conditions might be necessary, limiting the range of operation of the engine to certain syngas compositions, if high efficiency is to be achieved. The results show that the thermochemical recovery method provides the ability to improve the thermodynamics of the combustion process through dual-fuel combustion, while also recovering energy through the endothermic reforming process.