A new method for determining the parameters characterising elastoplastic deformation of two-phase material is proposed. The method is based on the results of neutron diffraction, which are analysed using the self-consistent rate-independent model of elastoplastic deformation. The neutron diffraction method (time-of-flight technique) was applied and the self-consistent model was used to predict the second order stresses in austeno-ferritic duplex steel. Calculations based on the model were successfully compared with experimental results for both phases of the duplex steel.