The present effort uses a combination of engine experiments and constant volume ignition delay calculations to investigate reformed fuel RCCI combustion. NOx emissions and efficiency are found to be a strong function of the engine operating parameters and soot emissions decrease with increasing fuel reforming due to a reduction in the mixing requirements of the diesel fuel. The impact of reformer composition is investigated by varying the syngas composition from 10% H-2 to approximately 80% H-2. The results of the investigation show that reformed fuel RCCI combustion is possible over a wide range of H-2/CO ratios. Replacing CO with H-2 resulted in a more reactive charge, decreased the combustion duration, and suppressed low temperature heat release. The suppression of low temperature heat release was explained through consumption of hydroxyl radicals by H-2.