Particle oxidation is one of the steps still not completely understood in combustion. Most of the approaches are based on semi-empirical reaction rates. Correct evaluation of oxidation is needed to predict the final emission of particles in diffusion flames. Fragmentation has recently been proposed to be a controlling step in determining global soot burn out as well as the size of particles emitted.The oxidation and fragmentation of soot particles is studied in a counterflow diffusion flames with in situ optical diagnostics, laser-induced incandescence and elastic light scattering. A sectional modeling approach is used to predict particle formation and burnout.Two counterflow diffusion flames have been chosen, a soot formation (SF) and a soot formation/oxidation (SFO) flame.Experimental data supported by model predictions show the role of fragmentation in controlling the burn-out and the size distribution of particles in flames. SF flame, where no soot oxidation occurs, shows large particles. By contrast in the SFO flame, the mean diameter of particles shows that when fragmentation is active coagulation is less effective, aggregates are hardly formed and primary particles with small size are mostly formed.