On heat treatment in air the solubility of MgO or TiO2, in Al23is too small to detect by lattice parameter shifts. The solubility of MgTiO3in Al2O3in air increased to the measured values, expressed as atomic fractions Mg:A1or Ti:A1of0.82 × lo‐2, 1.43 × 10‐2, and 1.75 × 10‐2at 1250°, 1650°, and 1850°C, respectively. In 1 atm hydrogen the TiO2solubility expressed as the atomic fraction Ti:A1 is 0.55 × lo‐2, 0.75 × 10W2, 1.15 × 10‐2, and 1.50 × 10p2at 1400°, 1500°, 1600°, and 1700°C, respectively. The increased solubility in H2was attributed to reduction of the titanium ion. The solubility of MgO in A12O3in vacuum(0.3μ)expressed as the atomic fraction Mg:A1 was measured as 1.10 × loW4, 3.00 × 10”4, 6.80 × 10–4, and 1.40 × 10‐3at 1530°, 1630°, 1730°, and 183O°C, respectively. These contents did not cause an observable change in lattice parameter, but a slight change was observed when MgO was diss