A method of combining computational fluid dynamics with a mathematical model for the particle dynamics has been applied to simulate experimental data from the synthesis of TiO2-particles in diffusion flames. Parameters of the coalescence kinetics are estimated by fitting the model predictions to the measured specific surface area of the product particles. The estimated kinetics can be used to predict the surface area and aggregate structure of the particles for a wide range of synthesis conditions. The regular equation for the rate of coagulation is modified to take into account the effect of dilution. The accuracy of the results, especially the degree of aggregation, i.e. the aggregate size, is highly dependent on the inclusion of this effect. When the dilution is accounted for, the predicted aggregate sizes (numbers of primary particles per aggregate) compare well with reported data based on small-angle X-ray scattering measurements.