Precipitation of barium sulfate has been studied in a rectangular flat reactor with jet mixing, both experimentally and with Computational Fluid Dynamics (CFD) modeling. The precipitation kinetics and mass balances were implemented in the CFD software (Fluent(R) version 4.25). The influence of the hydrodynamics on the quality of the precipitate has been Studied by changing the inlet velocity ratio while keeping the residence time and species concentration in the reactor constant. The CFD model was able to predict the influence of the inlet velocity ratio on the area mean particle size, the coefficient of variation of the particle size distribution, and the degree of conversion. Flow visualization experiments show that the simulated flow pattern was in good agreement with the experiments. A non-symmetric flow pattern was found, which was in agreement with the study of Murai e.a., 1989.