Abstract: Heat gelable droplets of a suitable liquid feed material are dielectrically heated to the gelling temperature by exposure to microwave radiation during free fall through a heating zone. The gelled microspheres thus produced are caught and collected in a washing liquid. The microwave radiation is provided through the use of a hollow cavity resonator. A vertical tube which is transparent to microwaves extends through the resonator to delimit the processing zone for the free falling droplets. Frequency control means and a reflected radiation measuring device are associated with the resonator and radiation generation means so that the operation of the system may be closely controlled.