A system employing eight NaI detectors has been developed for tracking particles moving in a three-phase fluidized bed reactor. One particle is tagged with typically 2 MBq of ⁴⁶Sc which emits high energy γ-rays. The system is calibrated by measuring the count-rates in the 8 detectors with the tagged particle placed at a number of locations in the reactor and then calculating the count-rates, using the Monte Carlo technique, at 19 200 locations. In a tracking experiment, data are accumulated in the multiscaling mode and the coordinates of the moving particle are calculatedby least-squares using the calibration map. With 30 ms counting intervals, the location of the particle is determined with a typical precision of 5 mm. The use of these data for determining velocity flow fields in multiphase reactors is illustrated.