Understanding the mixing of powders is vital because of its major impact on the performance of many unit operations and the behaviour of many products. However, little is known about the behaviour of particles within process equipment. The motion of particles in a vertical cylindrical mixer with two opposed flat blades was studied throughout the bed using positron emission particle tracking, this yielding the location of a tracer particle many times per second. At the wall the particles in the path of the blades were displaced upward, forming a heap, and then moved over the blade. Away from the wall a part of the upward flow moved away from the blades down the faces of the heaps. These particles also moved radially, producing three-dimensional recirculation regions. Recirculating particles took up to 60 blade passes to cross between the blades, compared with less than 3 blade passes outside these regions. The overall velocity field varied linearly with blade speed for the range of speeds studied. However, the size of the recirculation regions and the number of blade passes that the particles spent therein decreased as fill level was increased or as blade speed was increased.