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Knowledge of the long-term performance of adhesive connections is undoubtedly of paramount importance to enable their deployment in civil, mechanical, and other engineering applications. Over time, adverse environmental conditions can strongly influence the performance of adhesive joints leading to a progressive deterioration of their initial mechanical properties. The use of adhesive connections for secondary structures in offshore applications is a technology that allows for the rapid creation of structural members that, however, cannot ignore the influence of hydrothermal effects on mechanical performance due to environmental conditions. In this context, the investigation of the hygrothermal durability of adhesive connections was undertaken through an extensive experimental programme. More specifically, 130 cylindrical steel joints bonded with a commercially epoxy resin for structural applications were tested in Mode I using an Arcan-modified device. Prior to test, the specimens were placed in climatic ovens capable of combining the effects of temperature and humidity for approximately 320 days. In addition, the glass transition temperature, Tg, was assessed by employing the differential scanning calorimeter (DSC) technique to correctly define the experimental ageing conditions. The experimental results show how ageing conditions influence the mechanical properties of the epoxy resin investigated. Finally, some predictive formulations are proposed to calculate the loss of strength of adhesive joints over time.