A one step quenching and partitioning process was applied to a 0.2%C-2.0%Mn-0.5%Cr- 1.5%Si steel by quenching austenitised samples to several different temperatures below the experimentally determined martensite start temperature of 397 °C and isothermally partitioning them before quenching to room temperature using a quenching deformation dilatometer. These treatments yielded predominantly martensitic microstructures containing 5:6 vol.% to 7:5 vol.% retained austenite, as measured by x-ray diffraction. In each treatment, strong dilation was recorded during isothermal partitioning, with little indication of phase transformation during subsequent cooling to room temperature. This behaviour lends weight to the idea that an isothermal phase transformation occurred during partitioning, and that the final microstructure is a mixture of athermally and isothermally formed constituents. These results also suggest that the final microstructure of this steel is mostly formed before and during partitioning. © (2014) Trans Tech Publications, Switzerland.