A comprehensive and comparative experimental study was conducted to determine how the thermal ageing process, applied at different temperatures for different exposure durations, affects the mechanical properties and mechanical performance of the single lap composite joints bonded by adhesives with different properties. Initially, the glass fibre/epoxy composite joints were divided into seven different groups and each group was subjected to thermal ageing at -10 to 120 degrees C. Subsequently, specimens aged for 24 to 1440 hours were subjected to single lap shear tests to determine the change in their mechanical properties. As a result of this study, it is deduced that the maximum load carrying capacity generally decreases as the thermal ageing duration increases and take its lowest value at 120 degrees C, however, surprisingly the capacity of joints bonded with a ductile characteristic adhesive (DP 460) and aged at 80 degrees C shows an increasing trend as the ageing duration is prolonged.