Perovskite type Ba0.5Sr0.5Co0.8Fe0.2O3 (- delta) ceramics display nonlinear stress-strain behaviour upon uniaxial compression at room temperature. Step functional loading experiments show that the nonlinear strain response of the material is time dependent, partially reversible and depends on the oxygen vacancy concentration. In situ compressive stress-dependent synchrotron X-ray diffraction reveals that the nonlinearity is not related to ferroelasticity or a stress-induced phase transformation. The oxygen vacancy concentration and average spin state were determined from Rietveld analysis of the magnetic scattering found in the neutron powder diffraction data, indicating their role during mechanical loading. An oxygen vacancy migration model and a spin-state transition are proposed as possible mechanisms of nonlinear mechanical response. (C) 2016 Elsevier B.V. All rights reserved.