The comminution characteristic of the spiral jet mill is a super-position of the comminution as such and the sifting effect of the vortex dominating in the mill, thus resulting in a maximum of the load at mean specific milling energies. At higher specific energies the load in the mill decreases, leading to a broad particle size distribution. The residence time was measured with radioactive tracers and the load by the hold up of the mill. The decrease at low energies corresponds to instabilities of the slowly rotating vortex. The effects on the comminution are also evident from the ground particle size distributions. It is possible to monitor the milling process by measuring the pressure variation. Initial instability of the vortex corresponds to pressure variations, therefore the feed has to be reduced until stable operating conditions are achieved again. Based on the observations mentioned above, a scale up model which has already repeatedly proved effective in practise is introduced. It is based on the same cut size at the vortex finder diameter. The circumferential speed of the vortex has to determined experimentally, depending on the product properties.