The stability of a plasma-treated polymer surface is an important issue, but very often a surface rendered wettable by the treatment is found to revert to a less wettable state with time. The purpose of this work was to minimize the ageing phenomenon by stabilizing the surface layer via crosslinking using an inert gas discharge. The stability of the wettability, adhesion, and mechanical properties of treated polypropylene (PP) has been investigated by a comparative study of two different plasma treatments (i.e. an NH3 plasma and a He plasma pretreatment carried out before the NH3 plasma; He plasma is well known to crosslink polymer surfaces). The aluminium-polypropylene (Al-PP) interfaces present very different features depending on the gas treatment. The role of the treatment time has been pointed out and under our experimental conditions, a treatment time of the order of a few seconds seems to be an overtreatment leading to degradation of the adhesion and mechanical properties. A broad interphase was obtained for an NH3-overtreated PP, in contrast to the abrupt one formed when pretreated with the helium gas followed by NH3 treatment. Good correlations between wettability and mechanical properties with adhesion measurements were established.