gamma-based titanium aluminides are candidate materials for several high temperature structural applications. The orientation relationship, substructure and the interfacial structure of the massive product: matrix interface has been examined by transmission electron microscopy (TEM). The interfacial structure consisted of faceted, macroscopically planar interfaces, which were found to consist of regular arrays of ledges, no interfacial dislocations were observed. The substructure revealed a high density of planar defects such as twins, stacking faults and antiphase domain boundaries, such defects were also found in the vicinity of the interface. In another alloy, which had very few defects within the massive product, the expected orientation relationship between the product and parent phases was established. The low value of ledge height to interplanar spacing can explain the fast growth kinetics. The implications of these results on the understanding of massive transformations are discussed.