AbstractCarbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature (MS) by in-situ high-energy X-ray diffraction experiments. The time-resolved analysis of the phase fraction, austenite lattice parameter, and ferrite tetragonality offer the possibility of establishing phase-scaled consistent and precise carbon balances. In the studied steel, the carbon content in ferrite ranges between 0.1 and 0.15 wt.% after isothermal treatments, meaning that the available carbon for austenite stabilization is reduced. The stasis conditions are close to the T0′ criterion below 400°C but are not reached at higher temperatures. Our multi-step experiments also prove that the transformation below MS is very sensitive to the carbon content in austenite and auto-catalytic nucleation processes inherited from prior transformations (martensite and bainite at higher temperature). The bainitic transformation thus does not follow additivity rules.