The rotary valve is a widely used mechanical device in solids-handling industrial processes, but it is responsible for much attrition. Here, the attrition occurring in a rotary valve operating both as a stand-alone device and as part of a pneumatic conveying system was, for the first time, investigated. In the former case, attrition at three rotary valve speeds was consistent with the Gwyn correlation. For polypropylene, attrition was dependent on the number of rotations of the valve, but for PVC there was also a further rate dependent effect. In the latter case the Gwyn parameter was higher than those for higher air flow rates. This may be ascribed to a greater amount of material accumulating at the exit of the rotary valve. The attrition product tended to assume a narrow, elongated morphology. Attrition products were found to be in five basic shapes, these being dependent on the shearing action of the vanes in the rotary valve. The new concepts of shear frequency and attrition frequency are proposed. Over a wide range of attrition product sizes and shapes, the principles of attrition in the rotary valve were unified in a remarkably simple manner. As such, these concepts described well all findings and offered a characterisation of attrition in complex systems. (c) 2006 Elsevier Ltd. All rights reserved.