Critical issues for Mo5Si3, as an ultra-high-temperature material, include the understanding of the structure and the reduction of high anisotropy in the coefficient of thermal expansion (CTE). Dr. J. Schneibel, ORNL, is studying these alloys under OIT and BES funding using the HTML’s new Philips X'Pert Pro X-ray diffractometer system. This system provides parallel beam optics and a reduced temperature gradient in the furnace that improved the experimental precision dramatically over previous measurements. In Mo5Si3 the CTE along the c-direction is more than twice that in the a-direction. The anisotropy is due to an elastically more rigid basal plane and a higher anharmonicity along the c-axis. This higher anharmonicity along the c-axis is attributed to the existence of [001] Mo chains in the D8m structure of Mo5Si3. As these chain structures were either modified by alloying, or eliminated by structural modifi-cation (from D8m to D8l), we found significant changes in the CTE anisotropy. Additions of Nb up to approximately 40 at. % reduced the anisotropy. At higher Nb concentrations the CTE increased until the structure changed to that of Nb5Si3 and the CTE anisotropy decreased.  These findings have been reported in papers submitted to Intermetallics and The Physical Review B.