(\mathrm{Al}) high temperatures the (\mathrm{Ti}{2} \mathrm{O}{3}-\mathrm{MgTiO}{3}) system shows extensive solid solution formation with an order/ disorder transition between ilmenite- and corundum-type structures. At lower temperatures (\left(<1000^{\circ} \mathrm{C}\right)), there exists a large immiscibility dome extending from (\mathrm{Mg}{0.9} \mathrm{Ti}{1.1} \mathrm{O}{3}) to (\mathrm{Mg}{0.4} \mathrm{Ti}{1.5} \mathrm{O}{3}). Although (\mathrm{X})-ray peak intensities indicate a gradual change from order to disorder with composition, there is a discontinuity at the ilmenite/corundum boundary. The relationship between unit cell parameters and composition generally obeys Vegard's Law, apart from a deviation close to the (\mathrm{Ti}{2} \mathrm{O}_{3}) end member. Above (180 \mathrm{~K}) thermally activated conductivity is observed, with a discontinuity at the corundum/ilmenite boundary. At lower temperatures, a change in conductivity behavior occurs, with a linear relationship between (\log) resistivity and temperature being found up to (30 \mathrm{~K}) in samples with low Ti oxidation state. O 1993 Atindernic Press, the.