Nickel-based superalloys are employed in a variety of gas turbine components due to their excellent high-temperature properties. Unfortunately, several of these superalloys have proved to shrink during long-term service/ageing at intermediate temperatures (400-600°C). This study comprises an investigation of the ageing effects of two nickel-based superalloys from different classes of material: Haynes 230 and single crystal CMSX-4. The work is divided into three parts: I. Literature study. Haynes 230 and CMSX-4 were studied as well as reports and articles concerning shrinkage and microstructural changes owing to ageing in different nickel-based superalloys. II. Mechanical testing. The ageing effects on mechanical and physical properties were investigated in a series of experiments. The employed methods are tensile testing, impact testing, low cycle fatigue testing and hardness testing. III. Microstructural examination. The underlying causes to the property changes were investigated in a microstructural examination including carbide size measurements, chemical analysis of the composition and analysis of micro images taken with scanning electron microscope and light optical microscope. The results from the experimental testing show that the ageing has a marked effect on the properties of both CMSX-4 and Haynes 230. The low cycle fatigue properties of Haynes 230 are unaffected by the ageing while the yield and tensile strength and hardness are improved. The ductility is improved in both Haynes 230 and CMSX-4. The hardness of CMSX-4 was unaffected by the ageing while the tensile strength was slightly impaired. It was also found that the properties and the ageing effect depended on the single crystal direction. The study shows that the ageing has a significant effect on the precipitation of secondary carbides at grain boundaries in Haynes 230, which has a strengthening effect on the material. No distinct changes of the microstructure explaining the property changes in CMSX-4 were found. It is reported in other studies that shrinkage in other superalloys is caused by short and long range ordering based on Ni2Cr and Ni3Cr. The microstructural examination neither confirms nor refutes that the shrinkage and property changes are caused by this type of ordering. However, the results from the examination indicate that ordering formation is possible.