Twenty-five compositions in the binary system were reacted in the solid state at temperatures ranging from 1000° to 1670 °C. and examined by X-ray and optical methods. Four compositions near the eutectic were melted to glasses and treated by the quench technique at a temperature of 1575°C. The data obtained indicate solid solution of approximately 10% TiO2 in cristobalite at the eutectic temperature, and a relocation of the eutectic composition at 21.0 weight % TiO2. Solid solution of SiO2 in rutile seems to be even more extensive, but the limit was not definitely established. The solution of TiO2 in cristobalite causes the temperature of inversion to be lowered to 130 °C.; the magnitude of the volume change attending the inversion can be decreased to less than one-third the value for pure cristobalite. The physical properties of certain phases occurring in the system are discussed from the ceramic standpoint.