The solid solution Sn 0 . 5 Ti 0 . 5 O 2 was prepared by a high-energy ball milling process from the oxide mixture of SnO 2 and TiO 2 . After milling for 5 h, the sample was still a mixture of TiO 2 and SnO 2 although the grain size was greatly reduced. Ten-hour milling led to single phase Sn 0 . 5 Ti 0 . 5 O 2 . The effect of calcination on the phase formation and stability was investigated. The Sn 0 . 5 Ti 0 . 5 O 2 was stable up to 900 o C and separated into a Sn-rich phase (Sn 0 . 7 7 Ti 0 . 2 3 O 2 ) and a Ti-rich phase (Sn 0 . 1 7 Ti 0 . 8 3 O 2 ) at temperature above 1000 o C.