Dynamic mechanical thermal analysis (DMTA) techniques were used to investigate the moisture uptake and ageing of an ambient temperature cured epoxy used in the external strengthening of deteriorating concrete structures. Since the process is conducted under field conditions, cure does not progress completely prior to exposure to environmental conditions. Resin samples are immersed in deionized water at 23, 40, and 60°C, as well as in 5% NaCl and concrete based alkali solution at 23°C for periods up to 24 months. Diffusion coefficients increase with increase in temperature of immersion, but the maximum/equilibrium moisture content over the period of time is seen to be largely independent of temperature of immersion and type of solution. Glass transition temperature was found to decrease with increasing moisture uptake, with competing effects of cure progression and plasticization in the early periods of exposure, followed by hydrolysis and irreversible deterioration over longer periods of time. Splitting of the Tg of samples aged in deionized water and alkali solution indicate the formation of biphasic structures and drying of the network structure during DMTA. A biphasic structure indicating differently plasticized phases in the skin regions, which is different from the response in the bulk, is seen in samples immersed in salt solution. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1084–1094, 2007