A miniaturized scintillation probe has been used to follow the movement of particles tagged with radioactive isotopes Ir-192 and Tm-170. The small size of the probe (2 mm x 10 cm) has overcome two traditional powder monitoring problems, that of removing a representative sample and of causing gross bed disturbance by inserting a large probe. The miniature probe may be used in situ or on-line; it gives an extremely rapid response. Two examples of the use of the probe are briefly presented. It has been used to follow the axial mixing in a drum mixer and to follow the segregation of particles due to vibration. The probe has many potential uses for monitoring the progress of particles in pastes, doughs, slurries, and suspensions, in addition to dry powders.