Continuing our previous work, in which we analyzed the performance of Selemion ASV membrane (Asahi Glass Engineering) used to reduce the content of sulfate ions in concentrated brine from deep sea water (DSW) via a laboratory-scale electrodialysis (ED) apparatus, we analyze in situ the osmosis effect for Selemion CMV/ASV module (Asahi Glass Engineering) during ED process in this work. By recording the liquid volume in both concentrate and dilute tanks, it is found possible that the water flow across the membranes due to the osmosis effect could be estimated through the measurement of ionic volume flow. The analysis reveals that, when a higher DC voltage is applied, water flows from the concentrate to the dilute at initial stage of ED process, and it flows reversely at later stage: however if a lower DC voltage is applied, water flows from the concentrate to the dilute during whole ED process. Via a carpet searching process, a reasonable effective dynamic hydration number for hydrated ions is found as 3.5, and the water permeability of CMV/ASV module is estimated as 0.25214 cm/h. (C) 2011 Elsevier B.V. All rights reserved.