This paper focuses on the durability of carbon/polyamide 6 thermoplastic composites subjected to hydrolytic aging and tested under mode I static crack growth loading. In this study, aging was performed at temperatures ranging from 100 to 140 °C for durations up to 3 months in oxygen-free water. Results show that following wet aging the fracture toughness (energy release rate) decreases significantly, from an unaged value of 3.4 kJ/m2 down to values below 1 kJ/m2 after extensive degradation. This decrease was associated with a change from ductile to brittle behaviour, which allowed a critical molar mass M’c, to be determined for the first time on long carbon fibre reinforced composites.