In this paper the improvements of the thermal stability of cured epoxy resins in air as well as in various critical media like water, solvents, aerospace and automotive fluids are investigated. Upon modification with different addition levels of polysiloxane core-shell tougheners, the embrittlement upon thermo-oxidative ageing of the epoxy resin could be reduced significantly. Losses in strength and elongation caused by thermo-oxidative degradation of the polymer were minimized. The hygrothermal stability was increased significantly as well. It was found that the coefficient of thermal expansion (CTE) at temperatures above the glass transition temperature, increasing tremendously upon heat ageing, did not increase for a modified resin system. This enables the epoxy resin formulator to create systems for various electronic applications with a much better dimensional stability and an increased service life under harsh conditions.