A mathematical framework used to describe transformation toughening in zirconia-based ceramics is adapted to apply to transformation toughening in bulk metallic glass matrix composites. The method is applied to the Cu 47.5 Zr 47.5 Al 5 bulk metallic glass, showing that the low volume change of transformation in this alloy leads to negligible toughening via the proposed mechanism. An alternative mechanism for toughening is presented, whereby shear bands propagate more easily in the early stages of advance.