This three-part series of papers describes studies on water penetration into epoxy resins and the use of low- and high-frequency dielectric spectroscopy as a method of monitoring the health of adhesive bond lines in bonded epoxylgraphite composite structures. The first paper is concerned with an investigation of the effect of water ingress into the epoxy adhesive resin used in the manufacturing of the adhesive bond. This study reports a dynamic mechanical thermal analysis (DMTA) and gravimetric analysis of the effects of water uptake and interprets the data in terms of various processes that can occur within the adhesive. Surprisingly high values of water absorption it-ere observed in samples where the edges it-ere unconstrained and presented direct access to the fibre matrix for the moisture. The study demonstrated the effects of postcure and leaching on the sorption and desorption processes and provided background data, which are necessary for the interpretation of the study described in Parts II and III of this series of articles.