For more than a decade now, High Tenacity Polyvinyl Alcohol (HT-PVAl) yarns have been attracting a lot of interest in the geosynthetic community, particularly for reinforcement functions. PVAl interest principally lies in its excellent resistance to strongly alkaline or acidic environments combine with a high stiffness that makes it a great alternative material to classical reinforcement products. Today, several products manufactured with HT-PVAl yarns can be found on the market and their demand is boosted by the increasing use of soil treatments, recycled materials or low-quality fills with extreme pH conditions. Despite this rising attraction for PVAl, its characterization remains an open issue for the scientific community, as well as a challenge for the standardization organization that faces difficulties in specifying this material and assessing its long-term durability. In the first part of this study, we will focus our investigation on the physical behavior of HT-PVAl yarns particularly when exposed to water. Indeed, geosynthetic characterization are generally carried out in alkaline or acidic solution. Because of its chemical nature, PVAl has a strong affinity for water. Consequently, it is necessary to fully understand the mechanism of water sorption as well as its consequences on the physical structure and mechanical properties to correctly assess the durability of HT-PVAl yarns. Here, we will present a series of physicochemical and mechanical analyses aimed to determine the kinetics of water sorption, the plasticization effect of water, and its impact on mechanical properties (i.e tensile properties and creep). The authors will conclude this paper with the specific guidelines that should be respected when testing HT-PVAl yarns durability with regards of their water affinity.