This work investigates what phase transformations are taking place during a continuous heating as well as the influence of the solution temperature on the isothermal formation of martensite in a precipitation hardening semi-austenitic stainless steel. In previous studies in the stainless steel under investigation (12Cr-9Ni-4Mo-2Cu) only the isothermal mode of martensitic transformation has been experimentally detected. In this work it is shown that: 1) The AF temperature is located around 1040 K; 2) The χ-phase present in the initial microstructure dissolves above ~1323 K; 3) above 1448 K the formation of delta ferrite is promoted at austenite grain boundaries; 4) the kinetics of isothermal martensite formation is strongly accelerated with increasing solution temperature. The kinetics has been monitored in-situ at room temperature by using high resolution dilatometry. A semi-empirical dilatometry model is used to convert the dilatometry signal into volume fraction of martensite transformed. The results are briefly compared with previous kinetic measurements under the influence of strong magnetic fields.