The residual stresses in both thermosetting and thermoplastic single-fibre composites have been experimentally evaluated by means of an original technique based on the continuous monitoring of the fragmentation test performed at various temperatures. The difference between the strain at the break of a single fibre in air and one embedded in a polymeric matrix has been measured as a function of temperature. By considering the compressive fibre modulus this strain difference has been converted into fibre compressive stresses related to the matrix thermal shrinkage after curing of the samples. In fact, as the test temperature increased, the thermal compressive stresses decreased until a zero value was obtained, corresponding to a so called "stress free temperature", equal to the curing temperature for amorphous thermosetting matrix composites or equal to the matrix melting temperature for semicrystalline-thermoplastic matrix composites. The experimental results have been compared with data obtained from a theoretical model and a good agreement was found especially if the temperature dependence of the matrix Young’s modulus and matrix thermal expansion coefficient are accounted for in the computation.