The reverse martensitic (austenite) transformation temperatures (A
s) were investigated using a diffusion couple of PtTi and CoTi with a continuous compositional gradient. It was found that PtTi and CoTi form a complete solid solution of (Pt, Co)Ti at 1373K. Surface relief was formed by heating due to the austenite transformation. Judging from the formation of the surface patterns and the corresponding chemical compositions, A
s monotonously decreases with increasing Co content at a rate of -70K/at%Co, and A
s is estimated to be close to room temperature (RT) when the Co concentration is 15at%Co. Besides, micro Vickers hardness values measured at RT are minimized around 15at%Co.