An analysis is presented for the fracture of an adhesively bonded double-cantilever beam that fails with extensive plastic deformation of the adherends. The analysis permits a value for the toughness of the joint to be distinguished from the energy absorbed by the plastic deformation. Specifically, this value for toughness can be determined from post-fracture observations of the deformation and from a knowledge of the constitutive properties of the adherends. The analysis has been used to determine experimentally the toughness of plastically deforming joints formed using three different adhesives to bond a series of different thicknesses of aluminium and steel. In each case, it was found that, despite large differences in the extent of deformation, the value for toughness was dependent only on the materials used to form the joint. The toughness was independent of the thickness of the adherends.