This study was motivated by the fact that previous research on the structure and properties of Ti5Si3 showed unacceptably inconsistent results. The primary reason for these inconsistencies were due to interstitial contamination of Ti5Si3 by carbon, nitrogen and oxygen. Thus, this study measured the effects that these interstitial atoms have on some of the previously reported properties. These properties of Ti5Si 3Zx (Z = Be C, N or O; x = 0 to 1) include crystalline structure, thermal expansion anisotropy, electronic structure and bonding, and high temperature oxidation resistance. By using pure samples, accurate lattice parameters for pure Ti5Si3 were determined to be: a = 7.460+/-0.002 and c = 5.152+/- 0.002, and atomic positions to be: xTi = 0.2509 +/- 0.0005 and xSi = 0.6072 +/- 0.0005. As interstitial atoms are added to the lattice, Ti-Z separations contract sharply and Ti-Si separations expand, suggesting that bonds form between Ti and Z atoms at the expense of Ti-Si and Ti-Ti bonds. This agrees with LMTO-ASA calculations, which shows bonding between d(Ti) orbitals and p(Z) orbitals. Furthermore, carbon is shown to be most strongly bonded and oxygen least strongly bonded. These bonding changes measurably affect the thermal expansion anisotropy of Ti5Si3. In fact the ratio of the thermal expansion coefficient alpha a/alphac, as measured by XRD at synchrotron sources, decreased form three for pure Ti5Si3 to two for Ti5Si 3 With B, C or N additions. Additions of O or partial substitution of Si with Ge had only small effects on this ratio. Finally, long-term oxidation resistance was shown to be improved by carbon or excess Si but not by N or 0 additions. Parabolic rates of 4 xl0--5 mg2/cm 4/h were observed at 1000°C for Ti5Si3C 0.5 and Ti5Si3.2. These samples formed amorphous silica scales. However, Ti5Si3 formed a rutile-rich scale as well as TiN, TiSi, TiSi2 and Si layers beneath this scale. These sub-scale phases led to a large volume expansion and consequent disruption of the external oxide scale.