The use of close-coupled post injections of fuel is an incylinder soot-reduction technique that has much promise for high efficiency, heavy-duty diesel engines. Close-coupled post injections, short injections of fuel that occur soon after the end of the main fuel injection, have been known to reduce engineout soot at a wide range of engine operating conditions, including variations in injection timing, EGR level, load, boost, and speed. While many studies have investigated the performance of post injections, the details of the mechanism by which soot is reduced remains unclear. In this study, we have measured the efficacy of post injections over a range of load conditions, at constant speed, boost, and rail pressure, in a heavy-duty, optically-accessible research diesel engine. Here, the base load is varied by changing the main-injection duration. Measurements of engine-out soot indicate that not only does the efficacy of a post injection decrease at higher engine loads, but that the range of post-injection durations over which soot reduction is achievable is limited at higher loads. Optical measurements, including natural luminescence of soot and planar laser-induced incandescence of soot, provide information about the spatio-temporal development of in-cylinder soot through the cycle in cases with and without post injections. The optical results indicate that the post injection behaves similarly at different loads, but that its relative efficacy decreases due to the increase in soot resulting from longer main-injection durations.