A research program spanning over 30 years in DuPont has concentrated on fundamental understanding of the fluid mechanics in confined vortex fluid energy mills. Extensive internal reports and patents have documented numerous accomplishments. Direct visualization and measurement of velocity profiles, computational fluid dynamic modeling, in-line particle size measurement, and experimental study of gas molecular weight effect have resulted in optimized operation and improved energy efficiency. This paper will only discuss the effect of motive gases on the grinding performance. Helium, steam, air, and CO2 are studied as motive gases to provide a broad range of molecular weight that determines the gas specific sonic velocity and resultant kinetic energy. The experimental results indicated that a fully developed flow in the grinding chamber is needed for the built-in air classification to control the product top size. The shape of particle size distributions is not significantly affected by the properties of motive gases. The gas/solid ratio or the energy intensity does not determine the grinding limit, rather how fast the limit is reached. It is very clear, however, that the motive gas with the lighter molecular weight has the capability to reach a finer grinding limit. Simply, helium gas will be able to grind finer than steam, steam grinds finer than compressed air or nitrogen, and they are all better than CO2.