Abstract Introducing an “on-line” Coulter Counter measurement technique in a stirred fluidized bed of KAl(SO 4) 2·12H 2O crystals in order to observe the generated secondary nuclei leads to results. But even these results still do not give an answer to the complicated phenomena of birth and growth of the secondary nuclei. The results are the following: Secondary nuclei of sizes smaller than 2 μm are generated and the smaller the size the higher the number of the generated nuclei. Nuclei below about 3 μm seem not to be “effective”. The high number of particles in the unfiltered “pure” solution prior to the experiment does not grow in the sampling cells. The number of nuclei grown in the sampling cells corresponds to the number of nuclei above 3 μm counted by the Coulter Counter. There are particles growing more slowly in the sampling cells, but the number is much smaller than the number of particles below 3 μm counted by the Coulter Counter. This means some of these particles do not grow or they grow at very slow rates or something else happens to them. The higher the supersaturation the more nuclei are generated. The effect of supersaturation at high stirrer speeds is the least with the small nuclei, but the strongest with the larger nuclei sizes. That means more nuclei can be found in larger size ranges at high supersaturations and high stirrer speeds. The higher the stirrer speed the more nuclei are generated. The power number found for the supersaturation 1.3 fits well into the range of 1.3–1.6 given by former work.