The prediction of the properties of polymers in contact with water is important for many applications. In this paper two kinds of materials are investigated: an epoxy resin and its glass fibre composite. The measured complex dielectric constant is analysed with different effective medium models, taking into account the variation of water uptake according to time. Since the mixing laws do not describe the experimental results accurately, three new models are proposed, based on the assumption of a time-varying dielectric constant of the matrix during water immersion. They all give a better agreement between measurement and theory. In order to check their validity, these models are also applied to other data sets, found in the literature, and good correlations are found. Finally, we discuss the conclusions that one may derive from this kind of modelling, since many factors can influence the permittivity. Nonetheless, the new models are able to predict the variation of the dielectric constant for water immersion ageing.