Close lattice matching with a MgAl2O4 (001) substrate has been used to synthesize thin films of a rare spinel-like polymorph of MgO (beta -MgO) that is unstable in bulk form. The film is produced by spin casting an acetate-substituted Mg-ethoxide precursor, which forms a crystalline magnesium acetate phase at room temperature. Pyrolysis of most organic constituents occurs during continuous heating from room temperature to 400 degreesC and is followed by formation of phase-pure beta -MgO at T < 700 degreesC. Four-circle X-ray diffraction indicates that the film is highly oriented with the substrate lattice, with < 0.05 degrees tilt or twist between the (001) planes of the film grains and the (001) planes of the substrate. Kinetic and/or energetic constraints arising from the presence of a substrate are responsible for stabilizing beta -MgO: bulk powders formed from the same precursor yield only the usual rock salt MgO structure and, although the formation of some beta -MgO occurs on widely different substrate types, its volume fraction crystallographic orientation can be tuned by using closely lattice-matched substrate crystals.