Soot particles were synthesized in a rich C2H4/O2/Ar premixed flat flame in a low-pressure reactor. For particle synthesis the pressure in the reactor was varied between 40 and 50 mbar, while the velocity of the gas mixture was kept constant. Additionally the residence time of the soot particles in the combustion chamber was varied. The soot particle size distribution was determined by a particle mass spectrometer (PMS) attached to the flame reactor. TiRe-LII experiments for evaluation of mean particle size distributions were performed simultaneously in the reactor close to the PMS sampling location. The results of the TiRe-LII experiments were in good agreement with the results of the PMS measurements.