The compositional breakage equation is derived, in which the distributions of botanical components following milling of wheat are defined in terms of compositional breakage functions and concentration functions. The forms of the underlying functions are determined using experimental data for Outer Pericarp, Intermediate Layer, Aleurone and Starchy Endosperm generated from spectroscopic analysis of milled fractions of a hard and a soft wheat milled under Sharp-to-Sharp (S-S) and Dull-to-Dull (D-D) dispositions. For the hard Mallacca wheat, the Outer Pericarp, Intermediate Layer and Aleurone compositions mostly varied with particle size in similar ways, consistent with these layers fusing together as "bran" and breaking together, although with possibly a subtle difference around the production of very fine particles under D-D milling. By contrast, for the soft Consort wheat, Outer Pericarp, Intermediate Layer and Aleurone were distributed in broken particles very differently, particularly under D-D milling, suggesting a different breakage mechanism associated with differences in the mechanical properties and adhesion of the bran layers. These new insights into the nature of wheat breakage and the contributions of the component tissues could have implications for wheat breeding and flour mill operation. (C) 2016 Elsevier Ltd. All rights reserved.