The particle formation process during the precipitation of barium sulfate from aqueous solutions of barium chloride and sodium sulfate was studied experimentally and numerically. Particle formation was examined offline and in situ on timescales ranging from 55 ms to 1 s. Precipitation experiments were performed in a tubular reactor of adjustable length equipped with a flow-through cuvette. The mass fraction of the crystalline precipitate was followed time-resolved using wide-angle X-ray scattering (WAXS), performed at the synchrotron radiation facility ANKA of FZK Karlsruhe GmbH. A high degree of correspondence between numerical and experimental results was achieved using nucleation kinetics based on experimental findings in combination with diffusion-controlled growth kinetics in the population balance equation. Furthermore, the online measurements showed that the lattice ion ratio (educt ratio) has no influence on nucleation kinetics, which means that the adsorption of lattice ions on pre-nuclei has no influence on their respective interfacial energy.