Abstract : Aramid fibres are high-performance fibres proposed in geotextiles for treated ground reinforcement and in ropes and cables for marine applications. This work aims at answering the durability issues raised by a prolonged exposure in alkaline grounds (pH9-pH11) or in sea water, and at giving a better insight into the degradation mechanisms. For that purpose, accelerated agings in laboratory have been performed for three types of aramid fibres in different environments. First, the fibres have been characterized at the macromolecular, structural, morphological and macroscopic scales. Some strength losses mainly associated to chain scissions phenomena have been highlighted for Twaron 1000 fibres aged in tap water, in sea water, at pH9 and at pH11. However, the tensile modulus remains stable in these conditions. The same multi-scales approach has been adopted for Technora T240 fibres which appear to be very stable in these aggressive environments. Then, the influence of the finish on the fibres durability has been shown: not only the finish limits the surface abrasion between fibres, but it can protect the fibres towards hydrolysis as well. Finally, the comparison of the mechanical properties evolutions of polyesters and aramid fibres aged in an alkaline environment reveals that the Technora T240 fibres are well-suited for alkaline ground reinforcement, because of their high stability towards hydrolysis. The second part of this work is dedicated to the study of viscoelastic and viscoplastic behaviour of Twaron 1000 and Technora T200w fibres. The results indicate a significant influence of sea water immersion on the creep-recovery deformation, but the influence of aging on this behaviour is not obvious.