The moisture content sensitivity of Ethylene-Vinyl Alcohol (EVOH) copolymers is a major problem for food packaging applications. Thus, the relationship between the hydration mechanism at molecular scale and the properties is a critical issue for barrier properties improvement of EVOH films. The aim of this study was to determine material structure-functional property relationships and inter-property correlations. Firstly, mechanical, thermal, water sorption and oxygen barrier properties were determined for a wide range of water activity. The water sorption isotherm modelling with GAB equation, the evolution of the glass transition temperature applying Gordon-Taylor equation as well as the mechanical and O-2 barrier properties were studied. The results underlined the plasticization effect of water leading to a decrease of the cohesive energy density and a drastic loss of O-2 barrier properties of the EVOH copolymer from a critical point occurring when water activity was close to 0.5. Secondly, an analysis of the experimental water sorption isotherm, expressed as the number of water molecules sorbed per repeating unit in the amorphous phase, associated with an analysis of the evolution of polymer chains mobility allowed to establish a relationship between the hydration mechanism at molecular scale and the properties. It was determined that the amount of water sorbed molecules dramatically increased at a critical point, corresponding to the moment from which one water molecule was linked in average every nine amorphous PVOH units. This point corresponded to the saturation of the monolayer and the beginning of the clustering phenomenon.