The manufacturing of nickel-base superalloy forging for aero-engine applications requires a number of thermo-mechanical processing steps, which generate significant residual stresses that can result in distortion, uncontrolled deformation of the component during machining, or, when added to the in-service stresses, they will increase the crack initiation probability and hence reduce component life. Ageing treatments are applied to nickel-base superalloys in order to precipitate the strengthening phases that give to the alloys their remarkable strength over a wide temperature range. These isothermal treatments also result in a reduction of the residual stress level by means of thermal stress relaxation. In this work, the focus has been on the study of the thermal residual stress relaxation induced by the ageing treatment of wrought nickel-base superalloys. The aim of this work is to improve the understanding of the residual stress relaxation process and the different deformation mechanisms involved in the stress relaxation during isothermal treatments. This has been achieved by combining neutron diffraction with a new in-situ heating setup in order to track the elastic strain evolution in the centre of quenched disc shaped forgings of Inconel 718 and Udimet 720LI. This research has resulted in the implementation of an induction heating setup which was designed in order to serve in-situ residual stress analysis during isothermal treatments of large components at temperatures up to 1000°C. The in-situ analysis of residual stresses required the development of a standardised method which consists of monitoring the d-spacing evolution in stressed samples and in stress-free samples during isothermal treatments in order to determine the time dependent stress evolution. Stress calculations were performed using temperature dependent diffraction elastic constants which were determined experimentally for Inconel 718 and Udimet 720LI at high temperatures. The in-situ neutron diffraction measurements in water quenched disc shaped forgings of Inconel 718 of different thickness during isothermal treatments at 720°C revealed that the stress relaxation amplitude does not vary significantly with the initial stress distribution. However, the stress relaxation rate is strongly affected by the annealing temperature as stress relaxation through creep was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720°C and 750°C while no further stress relaxation occurred at 650°C. Most of the stress relaxation was found to occur during the heating stage as a result of a combination of plasticity and early stage creep relaxation. In-situ and ex-situ measurements exhibited good agreement on the amplitude of residual stress relaxation. It was found that a heat treatment at 750°C for 8h reduced the stresses by approximately 70% in Inconel 718 and only 20% in Udimet 720LI. For all in-situ experiments it was possible to fit the stress relaxation data by using logarithmic functions, which can now be used for validating and/or improving process models.