Age-momentum correlation (AMOC) positron annihilation technology was applied for the first time to composite nanofiltration membranes (NFMs) with zwitterionic terpolymers (TPs) to explain their high permeability and high salt selectivity. The NFMs showed an increase in the water flux from 32.6 to 46.3 L m(-2) h(-1) without sacrificing the salt selectivity when the zwitterionic monomer 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate (DMAPS) content was increased from 0 to 4.35 mol %. The influence of DMAPS content on membrane microstructure, water permeability, and salt selectivity was evaluated by means of AMOC positron annihilation studies. Positron annihilation results showed that the cross-linked TPs selective layer of NFMs was composed of two types of pores: free volume (based on tau(3)) with radius of about 3.5-3.1 angstrom and nanocavity (based on tau(4)) with radius of 16.6-46.6 angstrom. The performance of NFMs depends strongly on the free volume and nanocavity size. The effect of the zwitterionic DMAPS content on the free volume and nanocavity size in NFMs was as follows: the free volume was reduced as the molecular chains were arranged much closer to each other because of the increasing electrostatic interaction, and the nanocavity size was enlarged as the interfacial space was increased as a result of the increased molecular association. NFMs with larger nanocavities and smaller free volume have higher water permeability and higher salt (divalent/monovalent) selectivity.