The particle size distribution (psd) produced by breakage of wheat in the Perten Single Kernel Characterisation System (SKCS) was measured using sonic sifting, for a range of wheat varieties, kernel sizes and moisture contents. At moisture contents of 16% wb, the psd produced by the SKCS was very evenly spread over the range 106-3350 μm, with the average particle size much greater than would result from roller milling. Hard wheats gave slightly smaller average particle sizes in the broken material (this was unexpected and contrasts with First Break roller milling, for which hard wheats give larger output particles than soft wheats), but the variation in psd among different wheat varieties was surprisingly small. This indicates that the SKCS exerts a very positive breakage action on wheat grains, giving similar degrees of breakage to kernels of different hardness. The reported hardness index therefore depends primarily on the crushing force profile or energy to grind, and is not confounded by differences in the extent of breakage achieved. Kernel size similarly gave little difference in the output psd. The effect of increasing the moisture content from 9 to 17% wb was to increase the average output particle size greatly; moist kernels do not break so readily in the SKCS.