The separation of hydrogen in a thermochemical water decomposition process (UT-3 process) is discussed from the point of view of exergy analysis. In this process, hydrogen is produced by hydrolysis of CaBr and FeBr2, which are contained in reactors connected in series, and is separated from H2O and HBr at the outlet of the FeBr2 hydrolysis reactor at 220-560 degrees C. Hydrogen concentration is anticipated to be of the order of 5 mol %. On the basis of the permeation properties of H2O- and H-2-selective membranes developed in the preceding study, exergy losses for the hydrogen separation are calculated for plausible cases. The total exergy loss is smallest for a two-stage separation system : Steam is separated from a H-2-H2O-HBr mixture using a H2O-selective membrane with a minimum loss of pressure and is recycled to the CaBr2 hydrolysis reactor. The permeate is recompressed to the feed side of the second stage, and hydrogen is then recovered using a H-2-selective membrane.