Differential scanning calorimetry (d.s.c.) of composite samples made out of poly(ethyl methacrylate) (PEMA) and nylon fibres with a wide range of fibre volume fractions reveals several interesting features. A decrease in the glass transition temperature of the series as the fibre fraction increases, as well as a decrease of the specific heat increment at the transition are clearly detected; the glass transition (as determined by d.s.c.) even disappears in samples with high nylon content. The effect on the response thermograms of different ageing thermal histories is analysed with the help of a four-parameter model which predicts the qualitative features of the structural relaxation process. The parameters delivered by the fitting of the model to the observed behaviour suggest that the modified response of PEMA when acting as a matrix (when compared to its behaviour as a pure polymer) is caused by a shift of the equilibrium relaxation times with no accompanying widening of the relaxation time distribution. This supports an interpretation of the observed changes in terms of a bulk modification of PEMA in the composites, probably as a consequence of interactions of its molecules with nylon in the process of forming the composites.