AbstractA combination of a nozzle beam sampling system with a simple mass discriminator (mass range 50 to 105 mass units) is described which allows to determine the mass distributions and number densities both of positively and negatively charged soot particles in low pressure flames. The application to slightly sooting C2H2 ‐ 0, flames shows that a strong increase in ionization at the start of the soot forming zone is associated with a soot particle growth from a few hundred to about 2000 mass units. Ionizing soot particles can be distinguished from heavy hydrocarbon ions originating in the oxidation zone. Charged soot particles are smaller on the average than uncharged ones. The maximum number density of charged particles is strongly correlated to the amount of soot formed in the flame. The ionization potential of small soot particles is estimated to ∼5 eV.