In a gas-solid fluidized bed reactor tlie movement of the solids was investigated using24Na- and140La-marked catalyst, resp. For this purpose different grain size fractions were selected from the catalyst size spectrum and tlieir distributions in the fluidized bed were determined as a function of the product throughput. The marking procedure, the injection device, the electronic measurement technique, the test regime as well as the aspects of health physics to be considered for these investigations are discussed. Then a short description of the general procedure of the evaluation of the measuring results by means of two selected measuring points and of the encountered problems is given