The atomic structure of grain boundaries is of fundamental importance in determining their behavior and thus the properties of polycrystalline materials. There have been several studies (eg. refs 1-4) of the detailed structure of pure tilt boundaries using lattice-fringe and structure imaging. These studies have mainly been concerned with metals and semiconductors. There have been some reports on the detailed structure of grain boundaries in ceramic materials (eg. 5,6) but most effort there has been directed towards identifying and understanding the nature of the thin amorphous layer which appears often to be present. The present paper presents results of a new study on the structure of twin boundaries in an Mg-Al spinel. The spinel studied is the non-stoichiometric MgO.2Al2O3 in which the oxygen atoms form an fcc sublattice and the cations sit in the tetrahedral and octahedral intersticies.