TiO2 nanoparticles were synthesized in a premixed-flame reactor and in a microwave-plasma reactor. In both cases titanium tetraisopropoxide (TTIP) was used as precursor diluted in H2/O2/Ar. The particle size was measured in-situ by a particle mass spectrometer (PMS). The modeling of both processes was performed based on the reactive continuum equations including the population balance equations for the particle volume and the particle surface area. A monodisperse aerosol model was used, which considers nucleation, coagulation, coalescence, and surface growth. In both reactors, the particle properties were calculated and the influence of the precursor concentration on the particle size was investigated. The experimental mean particle size could be well predicted by numerical simulations.