Understanding thermo-mechanical fatigue (TMF) in superalloys has become more and more important due to the demand of increasing operation temperature of gas turbines to increase their efficiency. Very recent work has shown that the deformation and damage mechanisms in single crystal superalloys during TMF are very different from those traditionally reported to occur under creep or isothermal fatigue. Fundamental differences in the mechanical response due to ageing were also discovered and related to microstructural changes during TMF. Neutron experiments under in-situ heating are therefore proposed to study microstructural constituents and gamma-gamma¿ mismatch at different temperatures in CMSX-4 single crystals in virgin, long term aged and TMF tested conditions, respectively. The aim is to better understand the impact of long term ageing and TMF on the deformation behaviour of CMSX-4.