The ignition process of fuel reactivity controlled PCCI combustion was investigated using engine experiments and detailed CFD modeling. The experiments were performed using a modified all metal heavy-duty, compression-ignition engine. The engine was fueled using commercially available gasoline (PON 91.6) and ULSD diesel delivered through separate port and direct injection systems, respectively. Experiments were conducted at a steady state-engine load of 4.5 bar IMEP and speed of 1300 rev/min. In-cylinder optical measurements focused on understanding the fuel decomposition and fuel reactivity stratification provided through the charge preparation. The measurement technique utilized point location optical access through a modified cylinder head with two access points in the firedeck. Optical measurements of natural thermal emission were performed with an FTIR operating in the 2-4.5 μm spectral region. Measured spectra were indexed to engine crank-angle, thus enabling cycle-averaged, crank-angle-resolved in-cylinder spectroscopy. The measured spectra were compared to emission spectra from the HITRAN database for qualitative species formation. Experimentally measured spectra of fuel decomposition and the formation of aldehydes, water, and carbon dioxide were observed. The reaction extent of the measurements was calculated and compared to that of predictions from the CFD modeling. The modeling predictions used the KIVA 3v Release 2 code coupled with the CHEMKIN II solver and a reduced primary reference fuel mechanism. Comparison of the experimental and CFD modeling results showed good agreement of measured species both spatially and temporally. Reaction extent comparisons were used to provide quantitative evidence for spatially and temporally staged combustion established via in-cylinder reactivity blended charge preparation. The results demonstrated that, at the tested conditions, the charge preparation strategy delayed combustion by approximately 4-5 CAD in areas of lower fuel reactivity as compared to areas of higher fuel reactivity, thus extending the combustion duration. This establishes the well controlled ultra-low emission, high efficiency PCCI-type combustion with reasonable pressure rise rates described by references [14 and 15].