The particle formation of barium sulfate is examined in situ for reaction times from tens of milliseconds up to several seconds. Precipitation of the crystalline solid is performed in a tubular reactor of adjustable length equipped with a free jet cell. The transient development of the mass concentration of precipitated solid is determined using synchrotron radiation wide-angle X-ray scattering (WAXS). Precipitated suspensions are stabilized by off-line experiments to measure particle sizes without agglomeration and aggregation. The experimental data are compared to the results of population balance simulation calculations. Nucleation and growth kinetics are derived by theoretical approaches from the literature. It is found that barium sulfate particles grow by aggregation of previously formed particles. (C) 2015 Elsevier Ltd. All rights reserved.