Transformation-induced plasticity (TRIP) steels are becoming increasingly exploited for industrial applications because they show high strength and high uniform elongation (ductility). Despite this interest, the relative contributions of the various strengthening and straining mechanisms are often poorly understood. In this study, neutron diffraction is employed to quantify the contribution of different mechanisms to ductility and work hardening for a 0.25 wt.% C steel. Differences in stress strain response at different temperatures are related to the extent of the transformation of metastable austenite into martensite during deformation. At room temperature (RT) the transformation of austenite occurs gradually with straining, while at -50 degrees C the transformation occurs almost from the onset of loading. The associated transformation strain is reduced, comprising nearly half the total strain, lowering the apparent elastic modulus and explaining the relatively low work hardening compared to RT straining. By contrast, deformation at RI after pre-straining at -50 degrees C results in larger work hardening than for solely RI straining due to the higher martensite levels introduced at -50 degrees C. This is due to composite load transfer to the strong constituent from the soft matrix. The extent of the transformation is quantified as a function of strain at both temperatures as well as its effect on the work hardening and elongation. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.