The microstructural changes in titaniferous oxide materials during the chlorination reaction were studied under inert and reactive gas atmospheres (CO and Cl-2) using SEM and X-ray diffraction. Rutile undergoes grain growth and solid-state sintering at high temperatures, the extent of which increases with increasing temperature and decreasing partial pressure of oxygen. Chlorine, which is an electron acceptor, also promotes this process while it reacts with titanium cations, which greatly changes the surface structure with TiO and Ti3O5 as final phases on the surface. The surface structures developed during these changes have significant implications for the rutile chlorination kinetics. Titanium minerals such as titania slag, beneficiated ilmenite, and ilmenite undergo similar solid-state sintering and grain growth during the chlorination reaction, but the process is more complex.