The particle formation of two pseudo-polymorphic calcium oxalate hydrates has been examined in situ for reaction times from 15 ms to 30 s. Precipitation of the hydrates has taken place in a tubular reactor which was equipped with a flow cuvette. Using synchrotron Xray diffraction the mass fraction of the two pseudo-polymorphs in suspension could be determined. The experimental results have been compared to the results of simulation calculations which consider in parallel nucleation as well as growth of the two calcium oxalates by solving population balances for each hydrate. Parameters in the kinetic rate laws were derived from independent experiments. Excellent correspondence of the model simulations with the experimental results was found. The measurements support the idea that the initial ratios of the nucleation and growth rates mainly determine the phase distribution in the solid product. It should be possible to transfer the measuring technique presented here to the precipitation of other crystalline substances. (c) 2006 Elsevier B.V. All rights reserved.