Engine-out soot emissions are the result of a complex balance between in-cylinder soot formation and oxidation. Soot is formed in the diffusion flame, just after the lift-off length (LOL). Size and mass of soot particles increase through the diffusion flame and finally they are partially oxidized at the flame front. Therefore, engine-out soot emissions depend on the amount of soot formed and oxidized inside the combustion chamber. There is a considerable amount of work in the literature on characterization of soot formation. However, there is a clear lack of published research related to the characterization of soot oxidation. Thus, the main objective of the current research is to provide more knowledge and insight into the soot oxidation processes. For this purpose, a combination of theoretical and experimental tools were used. In particular, in-cylinder optical thickness (KL) was quantified with an optoelectronic sensor that uses two-color pyrometry. Experiments were conducted using a fully instrumented four-cylinder direct-injection light-duty diesel engine. Several sweeps of engine operating parameters such as injection pressure, injection timing, swirl and EGR rate that affect soot oxidation were performed. Time-resolved optical thickness (KL) along with air/fuel mixing metrics (Apparent Combustion Time) were used to estimate soot oxidation rates during the expansion process under different operating conditions.