The application of close-coupled post injections in diesel engines has been proven to be an effective in-cylinder strategy for soot reduction, without much fuel efficiency penalty. But due to the complexity of in-cylinder combustion, the soot reduction mechanism of post-injections is difficult to explain. Accordingly, a simulation study using a three dimensional computational fluid dynamics (CFD) model, coupled with the SpeedChem chemistry solver and a semi-detailed soot model, was carried out to investigate post-injection in a constant volume combustion chamber, which is more simple and controllable with respect to the boundary conditions than an engine.A 2-D axisymmetric mesh of radius 2 cm and height 5 cm was used to model the spray. Post-injection durations and initial oxygen concentrations were swept to study the efficacy of post-injection under different combustion conditions. Several factors that influence the evolution of soot were analyzed, including the distribution of temperature, oxygen concentration, and OH radicals. Additionally, newly developed analysis methods, which can quantify and visualize soot formation, soot oxidation, soot from the main-injection and soot from post-injections individually, were also used to provide more insight into the effects of post-injections.It was observed that both soot formation and soot oxidation are enhanced by post-injection. Soot formation is increased due to the larger amount of fuel injected, especially under low O2 concentration conditions where the high equivalence ratio favors soot formation. Two mechanisms of soot oxidation were seen which are affected by different factors: soot oxidation by O2 is more sensitive to elevated temperatures and soot oxidation by OH is enhanced due to the presence of OH radicals as intermediate products of post-injection combustion.