A study in an accelerated environment of the mechanical properties of glass-fibre-reinforced thermoplastic composites based on polyamide 66, poly(ethylene terephthalate) and poly(butylene terephthalate) is reported. Results showed a decrease of −90% to −50% in ultimate stress to failure and impact strength with ageing according to the nature of the matrix. Increasing the ageing temperature resulted in a faster degradation rate. Depending on the matrix nature, the mechanisms of ageing seemed to be different. Polyamide 66 and poly(butylene terephthalate) composites showed effects of both physical ageing through plasticisation and chemical degradation through chain scission. Poly(ethylene terephthalate) composites did not give evidence of any plasticisation. The extent of hydrolysis was quantified through end-group analysis and gel permeation chromatography measurements. Scanning electron microscopy observations showed that hygrothermal ageing reduced the effectiveness of the interfacial bonds.