Seawater desalination process with reverse osmosis membrane has been widely used to supply high quality water. Over the past decades, despite considerable performance improvements, our understanding of the interplay between membrane nanostructure and performance remains limited. Most RO membranes comprise a support polysulfone layer and a coating in highly cross-linked polyamide, which acts as active layer for the separation. The physical-chemical and structural characteristics of this active layer are thus critically important in controlling the membrane permeability as well as its selectivity. In here we propose to investigate the structure of PA active layer using NR at different stoichiometry and level of hydration to link molecular packing to transport properties.