We present an experimental and modelling methodology developed at LGRE research laboratory to characterize soot oxidation in the presence of different atmospheres (NO2, NO2/O2), simulating passive regeneration which occur in a Diesel Particulate Filter (DPF). Based on this methodology which aims at deriving the kinetic parameters for soot combustion, the thermal reactivity of different soot has been studied and compared. Soot were produced from a prototype Liebherr engine and on an engine dynamometer at R&D Moteurs company, under two engine cycles and for two different fuels. Small soot masses (15-30mg) were deposited on the quartz frit of the reactor and submitted to a gas flow (NO2 or NO2/O2), under different temperatures. The mole fractions of NO2, NO, CO2 and CO at the reactor outflow were measured by infrared analyzers. The soot oxidation rate and the sample remaining mass were deduced from CO/CO2 emissions.In order to determine the intrinsic kinetic parameters for soot combustion, we improved a model already built at LGRE and consisting of a coupled system of five partial differential equations which describe the spatial or the temporal evolutions of the gas mole fractions (NO2, CO, CO2 and NO) and of the sample mass. The numerical resolution of this model involves an optimization procedure which determines the best kinetic constants of this model comparing the experimental and the computed (at the reactor outflow) mass loss rates and gas mole densities at different temperatures.