Fire is one of the greatest industrial risks. In nuclear facilities this is complicated by the need to ensure the containment of airborne contamination at all times, in both normal and accidental situations. In the event of a fire, the soot particles emitted in the smoke may have a double impact, first on containment (clogging of filtration barriers in the ventilation network) and secondly on fire propagation, through the radiative properties of these particles. Consequently, a better understanding of their properties is needed, not only in the fire zones, but also in the smoke of diffusion flames encountered during a fire. Here, we present a study of the physical and optical parameters of soot particles sampled in the plumes of over-ventilated diffusion flames of acetylene, toluene and polymethyl methacrylate. For these three fuels, and relative to several global equivalence ratios, we have established the size distribution for the primary particles and soot aggregates, along with morphological parameters (prefactor and fractal dimension), soot emission factors and the mass specific extinction coefficient.