Abstract A 0.79 C–1.5 Si–1.98 Mn–0.98 Cr–0.24 Mo–1.06 Al–1.58 Co (wt.%) steel was isothermally heat treated at 200 °C for 10 days and 350 °C for 1 day to form a nanoscale bainitic microstructure consisting of nanobainitic ferrite laths with high dislocation density and retained austenite films. The microstructures of the samples were characterized by transmission electron microscopy and atom probe tomography. Despite the formation of nanoscale bainite with a high volume fraction of retained austenite in both steels, the ductility of both steels was surprisingly low. It is believed that this was associated with the formation of carbon-depleted retained austenite after isothermal transformation at 200 °C due to the formation of high number of Fe–C clusters and particles in the bainitic ferrite laths and carbon-enriched austenite after isothermal transformation at 350 °C.