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The deformation behavior of pseudoelastic NiTi shape memory alloys under multiaxial loading conditions is influenced by the evolution of anisotropic martensitic microstructures. We use structural data and elastic constants of B19' martensite calculated from first principles in a micromechanical model to simulate uni- and biaxial experiments with complex strain paths. The microstructural evolution in terms of volume fractions of different martensite variants and the effect of their elastic anisotropy are investigated in detail. The calculated macroscopic stress-strain data are in good agreement with experimental results reported. The simulations elucidate the relative importance of elastic and inelastic deformation mechanisms (twinning, detwinning and reorientation) for the multiaxial mechanical properties of NiTi. They provide a clear picture of the interplay between phase transformation, evolution of martensitic microstructures and macroscopic mechanical behavior. It is demonstrated that apparently small changes of variant volume fractions in twinned microstructures can significantly affect macroscopic stress states. © 2009 Acta Materialia Inc.