The flow of a particle through a hydrocyclone acting on water has been studied by positron emission particle tracking (PEPT). The positron-emitting radioactive tracer was F-18. It was found that the activity on an ion-exchange resin particle labeled with F-18 did not leach out into the water during the duration of an experiment. In the state-of-the-art PET camera it is shown to be possible to locate the centroid of the tracer particle with a standard deviation of only about 0.2 mm once per ms, making both the temporal and spatial resolution high enough to trace the particle in its very fast motion through the hydrocyclone. The design of the hydrocyclone was a modified Stairmand high-efficiency geometry with a long cone. The results are, among other things, shown as spatial tracks of the tracer particle as it moves through the hydrocyclone. Several interesting features were seen. The particle path, although the particle was much larger than the cut size of the cyclone, exhibited excursions into the inner, upwardly directed, part of the vortex giving rise to recirculatory loops. Moreover, at a particular position low in the cyclone, the particle exhibited a complicated flowpattern moving up and down repeatedly across this position. Careful analysis of the motion is presented, particularly of the motion low in the hydrocyclone, on basis of which it is made likely that this position represents the end of the vortex in the hydrocyclone. (C) 2011 Elsevier Ltd. All rights reserved.