Lattice parameter measurements using x-ray Guinier techniques found substantial negative deviations from Vegard's law in the NiO-MgO system at 600~ Negative deviations occur at 1000~ for samples containing small concentrations of CuO. Measurements of compositions in the CuO-NiO system allow prediction of the lattice constant of CuO having the rocksalt structure. The pressure at which CuO tenorite transforms to its rocksalt modification is estimated to be 335 kbar. An ordered compound, (Cu0.~sNio.65)O, shows a tetragonal distortion from cubic symmetry. Considerable diffuse scattering was observed in the powder diffraction pattern suggesting a variation in the local cation coordinations. This tetragonal distortion disappears rapidly in the ternary system. The ordered compound Cu2MgO~ is stable well into the ternary system, with strong evidence for Ni2+ substitution.Mixing properties in rocksalt oxide solid solutions are important in determining physical, surface, and catalytic properties. Oxide solid solutions, including NiO dissolved in MgO and (Ni,Mg)A1204 spinel phases, have been proposed for use in catalytic combustors both for improving combustion efficiency and for controlling oxides of nitrogen (NO=) (1-4).Until recently (5, 6) such solid solutions have been regarded as "simple" and easily characterized, both thermodynamically and structurally.Calorimetric studies of NiO-MgO have found sizable negative enthalpies of mixing throughout the solid solution series (5). When combined with previously determined activity-composition relations (7, 8), large negative excess entropies of mixing were also observed. This tendency toward order was supported by high resolution transmission electron microscopy studies of a sample (Ni0.4Mgo.6)O. Small domains, approximately 50-100A, were observed in several samples of this composition. The frequency of occurrence of the domains was found to increase with lower annealing temperatures. That the lower temperature samples show greater degrees of order was confirmed by further calorimetric studies (9).The ordering was explained by the covalency and directional character of the bonding of the Ni 2+ ion (5). A model was proposed in which the Ni2+ ion, when competing with spherically symmetrical ions of similar size (e.g., Mg 2+, Zn 2+) for the intermediate oxide anion, is able to allow a closer approach of the anions in one direction (5). This results in a distortion of the Ni 2+ polyhedra from a regular to a compressed octahedron. Such a mechanism leads to an ordered and stabilized solid solution.Cu 2+ and Ni 2+ have comparable octahedral ionic radii (10). Pure CuO crystallizes with the tenorite structure in which Cu 2+ is in square planar coordination. Partial solubility of CuO in MgO (11), CoO (12), and NiO (23) does occur. The solubility limits of the rocksalt solid solution are Xcuo ----0.21, 0.42, and 0.36, respectively, at temperatures near 1000~Intermediate ordered (CuxMu)Ox+~ compounds exist in all three systems (11-13). In CuO-MgO the intermediate compound, gfiggenite, ha...