Soot particles in the atmosphere significantly affect human health as well as the climate. In order to avoid soot emissions, exhaust gases are filtered and the particles are trapped on a filter. The particles are eventually removed by catalytic oxidation to CO2, a process that requires elevated temperatures. The reactivity, i.e. the temperature required for this process, is currently determined after sample collection by techniques such as temperature-programmed oxidation (TPO), Raman microscopy or high-resolution transmission electron microscopy (HRTEM), which are all time-consuming and expensive. Furthermore, a significant amount of soot has to be collected prior to the analysis. This study aims for a straight-forward method to characterize the oxidation reactivity of exhaust soot on-line. A differential flow-through oven-system was built to thermally treat soot and determine the oxidation losses in comparison to a reference channel. The performance of the system was assessed by propane soot as well as by K2CO3-containing soot, which feature largely different reactivities in earlier studies. The results of the new system are compared to TPO measurements by using the same temperature program. K2CO3-containing soot shows oxidation at significantly lower temperatures than pure propane soot. Photoacoustic spectroscopy (PAS) revealed a considerable temperature-dependent decrease of soot mass concentrations, starting at lower temperatures for the K2CO3-containing soot. Beyond that, the new setup can derive the oxidation reactivity from isothermal measurements, thus enabling on-line monitoring of the soot oxidation reactivity in transient systems. Isothermal measurements on a real diesel engine were conducted and correlated to different engine and exhaust gas parameters.