To understand the detailed processes occurring in stirred systems for particulate materials, experiments were conducted in a horizontal cylindrical mixer, 270 mm dia. and 650 mm long. It was stirred by a single long flat blade, varying the level of fill between 20 and 70% and the agitator speed between 20 and 45 rpm. The dispersion of blobs ill three regions-in the transaxial plane near the wall, in tire path of the blade, and in the bulk-suggested two classes of behavior. The dispersive behavior of particles was characterized by axial dispersion coefficients and root mean-square radial displacements, parameters commonly useful for process design. Both parameters showed that powder flow was independent of blade speed and controlled by the number of blade passes. Agitation was nonuniform in the transaxial plane with two regions of enhanced axial and radial displacement. These occurred near the free surface and immediately beneath the region where the blade arm was horizontal so that material readily slides off the blade. The axial dispersion coefficient was a maximum at a fill level of 40%, contributing effects arising from the free surface area of the bed and the conveying capacity of the blade.