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Boron removal is a significant Subject in seawater RO desalination field since World Health Organization(WHO)proposed the provisional guideline value to its concentration in drinking water because of the harmful property of boron to plants and animals.Today，SWRO plants in the world adopt various processes to meet the guideline value，and the processes are mainly classified into two groups such as single—stage RO process and multi-stage process.In order to develop the appropriate membranes for each process.we have researched with a point on the solute transport mechanism in RO membranes.In this study，some structural analyses were applied to polyamide composite RO membranes in order to investigate the relations between boron removal performance and structural parameters of RO membranes.According to the positron annihilation lifetime spectroscopy(PALS)analyses with positron beam method，it was found that the actual pore sizes in RO membranes were in the range of 0.56-0.70 nm in diameter and had correlation with the boron rejection rate.This result suggests that the pore size is one of main factors to determine the boron removal performance of RO membranes.Furthermore,the chemical structures of polyamide in R0 membranes were estimated by solid—state 13C—nuclear magnetic resonance (13C—NMR)spectroscopy analyses.Molecular dynamics simulations based on the estimated chemical structures showed that the calculated pore sizes in polyamide models were in the range of 0.6—0.8 nm in diameter,which is well agreed with the experimental result measured by the PALS.On the basis of this knowledge.Toray has succeeded in developing new RO membrane elements which suit for each stage of SWRO process for boron removal by strictly controlling membrane structures.