Changes in the structure of Ti5Si3 were measured by x-ray and neutron diffraction as carbon, nitrogen, or oxygen atoms were systematically incorporated into the lattice. Additionally, the lattice parameters and variable atomic positions of pure Ti5Si3 were determined to be a = 7.460 Å, c = 5.152 Å, xTi = 0.2509, and xSi = 0.6072. The measured trends in lattice parameters as carbon, nitrogen, or oxygen atoms were added to Ti5Si3 showed that most of the previous studies on supposedly pure Ti5Si3 were actually contaminated by these pervasive light elements. Also, oxygen and carbon additions were shown to strongly draw in the surrounding titanium atoms—evidence for bonding between these atoms. The bonding changes that occurred on addition of carbon, nitrogen, or oxygen acted to decrease the measured anisotropic properties of Ti5Si3, such as thermal expansion.