The influence of the mould material on the build-up of internal stresses during autoclave cure of a unidirectional carbon fibre composite material was investigated using embedded optical fibre Bragg gratings. Four flat moulds (i.e. aluminium, steel, carbon composite and carbon foam-based) with different thermal expansion coefficients were tested. It was observed that, from the beginning of cure, the carbon fibres were strained by contact with the expanding mould. The strain was larger when higher pressure was applied, although always below the thermal strain of the mould, but remained significant even under vacuum-bag cure. As a result, residual strains were found in samples cured on aluminium or steel moulds, whereas the samples cured on carbon composite and carbon foam moulds exhibited almost no strain after cooling-down.