Design influences the flow within a powder mixer but quantitative guidance is lacking. Here the performance of mixers of different geometry was compared using positron emission particle tracking. One mixer had six long flat blades; the other carried short paddles. With the former, blade angle and number of axial compartments had little effect on agitation in the transaxial plan but axial dispersion was enhanced by longer axial compartments. A loop of circulation was found below the shaft. For the short paddle device, the transaxial agitation was more uniform, with a lower mean angular velocity and narrower ranges of velocities. The mixing elements inhibited the formation of the loop of circulation. In both cases, the axial flow had a cellular structure created by the radial supports for the blades but the short paddles mixer showed more chaotic behaviour, the axial dispersion coefficient being typically five times higher and increasing with fill rather than decreasing as seen with the six-blade device. A rationale for the design of powder mixers is thus emerging.