A new scheme of reverse osmosis seawater desalination, which is called a brine conversion two-stage process, has been proposed by Toray Industries. In this process, a second-stage is added to a conventional RO unit, and the concentrated brine from the unit is further desalinated at the second-stage to increase the water recovery ratio over 40 up to 60%. The recovered energy from the first-stage brine is used to pump the feed to the second-stage up to approximately 10 MPa. For this purpose, a new membrane, UTC-80BCM, has been developed besides a conventional UTC-80. A test plant based on this scheme (210 m(3)/day) was built at Ehime factory, Toray, Inc. and was run under a wide range of operation conditions. In this paper, the results of these membrane properties and the mass transfer coefficients of each spiral-type element are reported. Using these data, we found that a computer program, which was developed previously, could simulate the behavior of the new scheme, when appropriate temperature and pressure corrections were adopted.