This study aims to assess the effects of ageing in boiling water on the transverse tensile mechanical properties of unidirectional commingled GF/PP composites, as well as the influence of the fibre/matrix adhesion on the water absorption and desorption mechanisms. For this purpose, different interfacial qualities were obtained by a modification of the fibre reinforcement sizing (polypropylene specific sizing, or no sizing), and of the matrix (with or without coupling agent). A very good retention of the mechanical properties was observed for those composites which had been treated so as to improve the fibre/matrix adhesion. It is also shown that the better the adhesion, the longer the water remained inside the composite material. This induced effect appears to be the counterpart of the protecting role against moisture of a strong interface. The interfacial interactions act as barriers both during absorption and during desorption. This leads to water molecule entrapment. Hence, the persistence of water trapped at the interfaces in the case of sized glass fibres composites leads us to recommend investigations on the long term effects on longitudinal tension properties, since the effects of zero-stress ageing are known to reduce fibre strength.