It is proposed that the fracture surface of delaminated specimens, and hence the critical strain energy release rate, is dependent on both the mode of fracture and the orientation of the plies on either side of the delamination with respect to the propagation direction. Recent fractographs of Mode III delamination surfaces obtained by the authors have reinforced the idea that the properties, GIIc and GIIIc, are structural rather than material properties for composite laminates. In this study, the relationship between the mode of fracture, the ply orientation, and the apparent interlaminar toughness has been explored. Standard double-cantilever-beam and end-notched flexure tests have been used, as has the newly developed Mode III modified split cantilever beam test. Delaminations between plies of various orientations have been constrained to the desired plane using Teflon inserts running along the entire length of the specimen. As well, scanning electron microscopy (SEM) fractography has been extensively used so that measured energies can be correlated to the surface deformation. While fractographs show that Modes II and III share common fractographic features, corresponding values of Gc do not correlate, and it is shown that the large plastic zone of fractured Mode II specimens eliminates any comparison between the two. In contrast, Mode I delamination is found to be independent of the orientation of the delaminating plies.