Eleven granular materials of various sizes and internal structures were tested in an annular attrition cell that models the breakage occurring in a failure zone of a deforming powder. The cell permits the stress and the strain to be determined and controlled. For none of these materials were the traditional laws of comminution correct, nor was a first-order kinetic model of degradation generally found. A description of the rate of attrition following an empirical form due to Gwyn was the most effective. The product size distribution was well described by the Schuhmann formulation and this in turn led to a means of modifying the Gwyn approach. Using an experimentally measured parameter to describe the influence of normal stress on attrition rate, the data for a given sample could be unified.