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Cross-linking polymerization initiated by high energy radiation is a very attractive technique for the production of high performance composite materials. This method in fact offers many advantages compared to conventional thermal curing processes, due to the possibility to operate at mild temperature and in short time, limiting both energy and time consuming [1-2]. High performance composite materials mainly consist of epoxy resins as matrix and carbon fibers as reinforce, due to their excellent properties in terms of thermal and mechanical resistance. An important requirement of such systems for structural applications is their ability to maintain the properties within a fixed range during their operative life, i.e. to show significant resistance to the external aging factors. Among them, one of the recurrent ageing conditions is hydrothermal ageing, due to both temperature cycles and water absorption-desorption [3]. These conditions induce some important modification in the thermo-mechanical behaviour of epoxies, including plasticisation and degradation of the network, with a general lowering of the glass transition temperature, modification of the strength and fracture toughness, and swelling which may induce important localized internal stresses [4]. All these phenomena are very difficult to investigate, due to the several mutual effects occurring, and to a lack of reliable and simple measuring techniques. Some of the authors have recently proposed a new approach to evaluate the transitory swelling/un-swelling stresses, by means of Photoelastic Stress Analysis (PSA) [5]. This technique provided a robust method to evaluate the material in terms of its ability to swell and to develop internal stresses, correlating this property to the kinetics of water uptake and to the thermal and mechanical properties of epoxies. In this work electron beam curing was used in order to obtain two epoxy resin systems with very different cross-linking features in terms of both crosslinking homogeneity and water chemical affinity. The different stress behaviour of the irradiated systems upon water absorption/desorption was investigated by means of PSA and the results were interpreted on the basis of the comparison to those of the same monomer thermally cured. The results evidenced the strict correlation between network structure of the resins and swelling induced internal stresses, allowing to evaluate the evolving stress field and providing a different point of view on the investigation of differently crosslinked structures associated to water diffusion.