Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA-FTi.r.) has proved to be a useful tool for finding out about quantitative structural changes in glass-fibre-reinforced poly(ethylene terephthalate) (PET) obtained by compression moulding and subsequently annealed. Analysis of the photoacoustic parameters indicates that the photoacoustic characteristics of the composite are very similar to those of the isolated matrix. It was demonstrated that glass-fibre-reinforced PET satisfies the same two-phase conformational model as the matrix. In contrast to the annealed matrix, the primary and secondary isomerizations cannot be detected in the composite. Therefore, the thermal treatment on the composite samples does not show further conformational changes. As a result, the manufacturing process seems to determine the composite microstructure, and its effect is quite similar to that observed in annealed matrix at the highest temperatures. Consequently, it can be considered that the trans isomer content in the amorphous phase is negligible in the composite.