Microstructural evolutions during heat treatment of steel have been generally studied employing post-processed (ex-situ) characterisation methods. However, the data obtained cannot describe the kinetics of the transformation processes (in-situ) itself. With several microstructural processes happening simultaneously during an isothermal treatment below Ms, an in-situ characterisation method which can provide information on the microstructure development is needed. In this regard, Synchrotron X-ray Diffraction (SXRD) method is found to be a suited method. This thesis aims to study the microstructural developments during isothermal treatments below the Ms temperature by in-situ SXRD measurements in a model alloy of composition Fe–0.2C–3.51Mn–1.52Si–0.25Mo (wt.%). Results from this study show that during the isothermal treatment below Ms, bainite was formed right after the isothermal treatment started, which supports previous dilatometry experiments. During the isothermal treatment below Ms, four processes are suggested to occur simultaneously: (i) carbon redistribution processes from martensite, (ii) formation of bainite at the martensite/austenite interface, (iii) formation of bainite at the austenite/austenite interface, and (iv) carbon redistribution processes associated with bainite formation.