The measurement of the thermo-mechanical properties of cubic Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) with ring-on-ring test and depth-sensitive micro-indentation testing revealed an anomaly between 200 degrees C and 400 degrees C. In particular, the stiffness decreases strongly between room temperature (RT) and 200 degrees C, and subsequently increases again to a stable value at 300 degrees C. Fracture stress and toughness show a qualitatively similar behavior. The fracture stress decreases up to 400 degrees C, before recovering between 500 degrees C and 800 degrees C. XRD verified that BSCF remains cubic in the relevant temperature range. Hence the anomalies are attributed to the transition of Co3+ spin states, which is experimentally confirmed by susceptibility measurements. The fracture surfaces of the specimens were not affected by the anomaly between RT and 700 degrees C, and only a purely transgranular mode was observed. (C) 2009 Elsevier B.V. All rights reserved.