Several Ti5Si3Zx compositions (Z = B,C,N,O and 0 ≤ x ≤ 1) were arc-melted, and powder x-ray diffraction indicates the materials maintain the Mn5Si3-type structure of undoped Ti5Si3. Single crystal x-ray diffraction verifies that the ternary additions occupy the normally vacant interstitial sites in the Mn5Si3-type structure. Interstitial nitrogen and oxygen also significantly increase the density of Ti5Si3;Coefficient of thermal expansion (CTE) of Ti5Si3 was measured by the method of high temperature x-ray diffraction. CTE along the c-axis is substantially larger than along the a-axis ([alpha] a = 8.68 ± 0.14 and [alpha] c = 20.4 ± 0.4 ppm/K from 298-873 K). This contributes to severe microcracking in coarse-grained Ti5Si3 with an average grain size 10-20 [mu]m. Reducing the grain size to 1-2 [mu]m eliminates microcracking and nearly doubles hardness to 17.1 ± 0.7 GPa. CTE anisotropy for Ti5Si3C0.85 decreases ([alpha] a = 9.43 ± 0.29 and [alpha] c = 17.9 ± 0.6 ppm/K from 298-873 K);The isothermal oxidation resistance of hot isostatically pressed Ti5Si3Zx was measured from 700∘-1306∘C in flowing high purity air. The oxidation resistance of Ti5Si3 in air is adequate up to about 700∘C, but by 900∘C, substantial mass gain occurs with rapid linear oxidation kinetics;The addition of boron, carbon, and oxygen dramatically improve the oxidation resistance of Ti5Si3. Carbon-doped Ti5Si3 maintains protective behavior to about 1200∘C and forms a thin, continuous duplex scale. A continuous external rutile layer indicates that outward diffusion of titanium to the scale/gas interface is still fast enough to support formation of the continuous rutile layer. Boron and oxygen additions also improve oxidation resistance to about 1100∘C. Both materials contain thin, adherent continuous silica scales with discrete rutile particles at the scale/gas interface. At 1306∘C, protective behavior is generally lost, but Ti5Si3O0.25 maintains protective behavior with a mass gain of about 1.1 mg/cm2 after 240 hours. The extent of interstitial bonding in oxygen-containing Ti5Si3, in contrast to carbon- and boron-containing Ti5Si3, is evidently large enough to reduce outward Ti diffusion and thus suppress formation of a continuous external rutile layer. Nitrogen-bearing Ti5Si3 behaves almost identically to undoped Ti5Si3 with little improvement in oxidation resistance.