Scanning electron microscopy and energy dispersive spectroscopy analysis were used to characterize the interfacial reaction layer for a system comprised of commercially pure titanium cast into an incompatible silica-containing investment mold. This layer was compared to bulk mold material. Results of this comparison revealed that cristobalite was converted into a vitreous glassy silica phase to a depth of about 500 μm. Both a light and a darker appearing vitreous phase was identified. The former was associated with coalescence of original cristobalite particles and the latter with degradation of the magnesium phosphate matrix. The silicon-to-oxygen ratio of the light phase was significantly higher than that of darker phase. Both phases were contaminated with magnesium, phosphorus, chlorine, calcium, and titanium. Silicon was detected only short distances within the titanium substrate. The interfacial layer formed when molten titanium was cast into a quartz-containing investment, which is compatible with titanium castings, displayed similar microstructure to the bulk mold material. The violent reaction between cristobalite and molten titanium could have been due to contamination of cristobalite, either occurring naturally or from setting and heating of the phosphate matrix. This resulted in a lowering of the melting temperature of cristobalite and inducing the formation of metal slags and vitreous glassy silica with various stoichiometrics. Molten cristobalite also emitted its gaseous contaminants that reacted with molten titanium both embrittling the metal and providing the source of the porosity.