Two-Color imaging optics were developed and used to observe soot emission processes in a modern heavy-duty diesel engine. The engine was equipped with a common rail, electronically-controlled, high-pressure fuel injection system that is capable of up to four injection pulses per engine cycle. The engine was instrumented with an endoscope system for optical access for the combustion visualization. Multidimensional combustion and soot modeling results were used for comparisons to enhance the understanding and interpretation of the experimental data. Good agreement between computed and measured cylinder pressures, heat release and soot and NOx emissions was achieved. In addition, good qualitative agreement was found between in-cylinder soot concentration (KL) and temperature fields obtained from the endoscope images and those obtained from the multidimensional modeling. The imaging results were used in conjunction with modeling to explain why split injections are beneficial to reduce soot emissions and also why they may under retarded timing conditions increase soot emissions leading to the experimentally observed “soot catastrophe.” The reason for the dramatically increased soot at retarded timings is shown in the images to be due to poor ignition and combustion of the late injected fuel pulses in multiple injection schemes. The present modeling results also suggest a feedback mechanism where high in-cylinder soot levels can lead to high radiative heat transfer which promotes fuel vaporization. This, in turn, results in more locally fuel rich combustion which enhances the formation of soot.