We report a detailed study of the strain hardening behavior of a Fe-16Mn-10Al-0.86C-5Ni (weight percent) high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) during uniaxial tensile deformation. The dual-phase (γ-austenite and B2 intermetallic compound) HSSS possesses high yield strength of 1.2-1.4 GPa and uniform elongation of 18-34%. The tensile deformation of the HSSS exhibits an initial yield-peak, followed by a transient characterized by an up-turn of the strain hardening rate. Using synchrotron based high-energy in situ X-ray diffraction, the evolution of lattice strains in both the γ and B2 phases was monitored, which has disclosed an explicit elasto-plastic transition through load transfer and strain partitioning between the two phases followed by co-deformation. The unloading-reloading tests revealed the Bauschinger effect: During unloading yield in γ occurs even when the applied load is still in tension. The extraordinary strain hardening rate can be attributed to the high back stresses that arise from the strain incompatibility caused by the microstructural heterogeneity in the HSSS. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (65 refs)