Effects of cation order-disorder on the pseudobrookite-type MgTi 2 O 5 structure have been studied with single-crystal X-ray diffraction on five synthetic crystals (labeled P600, P700, P800, P1000, and P1400) annealed at 600, 700, 800, 1000, and 1400°C, respectively. The disorder parameters (X ؍ Ti in the M1 site) determined for five samples range from 0.070(5) to 0.485(5). Unit-cell dimensions a and c increase with X, whereas b decreases. The decrease in b with increasing cation disorder results primarily from the marked reduction in the O3-O3 distance within the M1 octahedron, whereas the variation of a and c is controlled by the size of the M2 octahedron. From P600 to P1400, the mean M1-O length shortens by 2.35%; in contrast, the mean M2-O distance lengthens by 1.16%. As a consequence, the M1 and M2 sites have a nearly identical octahedral volume in P1400. With increasing cation disorder, the degree of distortion of the M1 octahedron is reduced significantly, while that of M2 remains essentially unchanged. Cation disorder is also accompanied by straightening of the bands consisting of the M1 and M2 octahedra. There is a close correlation between atomic isotropic displacement factors (B iso ) and cation order-disorder: B iso factors of all atoms in more disordered samples are larger than those in less disordered ones. 1998 Academic Press FIG. 1. Crystal structure of pseudobrookite-type MgTi O : (a) viewed along c, showing the bands of the M1 and M2 octahedra, and (b) viewed along b, showing the edge-shared M2 octahedral layer.