Dispersion is a very important step in powder handling. Microgravity environments were applied to study, the process of powder dispersion with an orifice for low transportation gas velocities and for widely, sized particle agglomerates. Particle-size distributions were measured for particles after passing through the orifice, and the results were compared with the size distributions of primary particles. Data analysis showed that the main factor determining the dispersion efficiency is the inertial parameter, Two approaches are proposed for estimating the inertial parameter in a wide range of Reynolds numbers.