The martensitic transformation behaviour of the metastable austenite phase in low-alloyed transformation-induced plasticity (TRIP) steels has been studied in situ using high-energy X-ray diffraction during deformation. The austenite stability during tensile deformation has been evaluated at different length scales. A powder diffraction analysis has been performed to correlate the macroscopic behaviour of the material to the observed changes in the volume phase fraction. Moreover, the austenite deformation response has been studied at the length scale of individual grains, where an in-depth characterization of four selected grains has been performed, including grain volume, local carbon concentration and grain orientation. For the first time, a high-resolution far-field detector was used to study the initial and evolving structure of individual austenite grains during uniaxial tensile deformation. It was found that the austenite subgrain size does not change significantly during tensile deformation. Most austenite grains show a complete martensitic transformation in a single loading step.