The formation of single-crystal Mg2TiO4 films and large-grained, multiply positioned MgTiO3 films by solid-state interface reactions is investigated by transmission electron microscopy, selected area electron diffraction, electron probe microanalysis, Rutherford backscattering spectrometry, and other methods. Stoichiometric single-crystal Mg2TiO4 spinel films grow by a topotaxial reaction between the MgO substrate and a TiO2 vapor. A submicron-marker experiment revealed the cations to be the diffusing species. The films are free of observable lattice defects, except for a network of cation antiphase boundaries. Two new preparation methods are presented that enable edge-on solid–solid reaction interfaces to be investigated by in situ observation in the high-voltage electron microscope. Periodic lines of contrast at the semicoherent Mg2TiO4/MgTiO3 interface are interpreted in terms of dislocations, which probably restack the oxygen sublattice of MgTiO3 to that of spinel. The single-phase MgTiO3 films obtained show a regular eightfold-positioned structure, partly due to grain growth and reorientation processes. Interface strain is assumed to determine whether Mg2TiO4 or MgTiO3 is formed as the final thin-film phase during the solid–solid reaction.