The predominance of epoxy matrices in the manufacture of high performance carbon fibre composites for aerospace structural applications is expected to continue well into the next century. Tetraglycidyl 4,4’-diaminodiphenylmethane (TGDDM) is an epoxy matrix system favoured currently for high performance aerospace structural applications. However, absorption of environmental moisture and its damaging effect on composite performance has been recognised as a major problem with epoxy matrices. This study describes the properties of a series of low water affinity epoxy matrix resins based on halogen-substituted TGDDM and the improvements shown in composite performance after oxidative and hygrothermal ageing compared with the baseline TGDDM matrix. Two other proprietary epoxy systems offering high temperature or improved hot-wet performance were also examined together with a proprietary low water affinity curing agent. The halogen-substituted TGDDM systems offered improvements in bulk resin properties which were also translated to derived composite properties with significant improvements in the retention of hot/wet ILSS and Tg compared with TGDDM. The effects of accelerated thermo-oxidative ageing followed by hygrothermal damage mechanisms which may be applicable to service operating conditions were also investigated. The Cl2-TGDDM system in particular can be regarded as a highly promising matrix system with improved hot-wet properties and features prominently in the experimental programme reported in this thesis.