The binary tetragonal intermetallic compound Mo 5 Si 3 has significant thermal expansion anisotropy (TEA) with the coefficient of thermal expansion (CTE) of the c-axis (α c ) more than double that of the a-axis (α a ). Since boron has been shown to improve the oxidation stability of alloys near this composition, we explore the effect of adding boron on the thermal expansion anisotropy of Mo 5 + y Si 3 - y B x (T1) and subsequently the adjoining phases in thermal equilibrium. Powder X-ray diffraction was conducted at high temperatures using high energy (>40 keV) synchrotron X-rays. All of the intermetallics in this system (Mo 5 SiB 2 , MoSi 2 , MoB and Mo 3 Si) have moderate CTE values ranging from 7.17 ppm/ o C for the cubic Mo 3 Si to nearly 14 ppm/ o C for the c-axis of Mo 5 Si 3 B. The nearly isotropic TEA for Mo 5 B 2 Si was confirmed. The variability of the TEA of Mo 5 Si 3 B from 1.83 to 2.43 is explained in terms of the site occupancy and the change of the inter-atomic spacing of the Mo atom chains along the c-axis.