The long-term water absorption behavior of UD and +/- 45 carbon fiber/977-2 epoxy composite and unreinforced 977-2 epoxy resin has been studied at different environments with various temperatures and relative humidities. Fickian behavior was noticed initially followed by a slight and continuing increase in saturation with time, a behavior that continues for at least 4 years. Fickian saturation and diffusion coefficients of all materials were derived using graphical method. Initial Fickian equilibrium moisture contents were found not to be dependent on temperature but the long-term increase in solubility increased with temperature. Absorption rates showed strong dependency on temperature and relative humidity. The activation energy values for all materials in the initial stage found to be much higher (40 kJ/mole) than the long-term stage (10 kJ/mole). E-a values of the neat resin were observed to be slightly lower than the composites. The slow continued weight gain in the long-term absorption experiments has been seen to be coupled with an incomplete drying and a higher diffusion rate on drying and subsequent re-conditioning. The possible reasons and mechanisms for the long-term gradual increase in moisture absorption was further investigated. It was found that molecular relaxation is the most likely dominant mechanism. POLYM. ENG. SCI., 58:2175-2184, 2018. (c) 2018 Society of Plastics Engineers