We proposed a new approach to characterize the reverse osmosis (RO) permeability in conjunction with the macromolecular structures and inherent polymer properties for crosslinked and linear model aromatic polyamides. Aromatic polyamides were synthesized via interfacial reaction of phenylene diamines (p-and m-phenylene diamines) with either tri-functional (trimesoyl chloride) or di-functional (terephthaloyl and isophthaloyl chlorides) acyl halides. The relaxation properties obtained by cross polarization/magic angle spinning (CP/MAS) C-13 NMR spectroscopy, in conjunction with the chemical structures, built a bridge between the specific polymer properties and the RO performance of the aromatic polyamides. The spin-lattice relaxation time in the rotating frame, T-1 rho and hence chain mobility seemed to be an important parameter to control the RO membrane permeability. The longer T-1 rho resulted from the crosslinked aromatic polyamides, in which presence of the crosslinking retarded local polymer motion, and a lower water flux resulted. In contrast, the shorter T-1 rho for the linear crosslinked polyamides played a significant role for the higher water flux.