＇＇Process technology needs information on the running process and needs data for control. In crystallization-processes the knowledge of the process conditions has special importants for control in order to achieve the desired products (crystal size distributions). Today used measurement techniques have problems to detect very small particles (below 1 mu m) under process conditions. This examination should show whether the tested equipement could fill this gap, especially to measure in-line at the same time particle number concentrations in wide ranges as well as in wide size ranges (below 1 mu m), too. The results document that the tested measuring instrument (＇Three-wavelengths-extenuation＇) can fulfill the aimed for targets to measure particle distributions in-line in solution crystallization in a quite promising manner.＇＇