The batch concentration of salts of carboxylic acids by batch concentration reverse osmosis is discussed in this paper. The mathematical treatment is a fairly simple one that is based on the 'mosaic' theory and methods of finding the membrane parameter (sigma) for the separation is outlined by suitable integration of the variables involved. The process has two main functions; to dewater the feed and retain as much of the salt in the feed as possible. For this purpose an efficiency parameter is defined which can be simply evaluated. The paper gives a comprehensive coverage of the properties of the model and the integration of the two differential equations is a fairly simple process. It is recommended that tests with pure water and NaCl solution should be carried out in parallel with tests on the salts to obtain information on the general operation of the module. (C) 2002 Elsevier Science Ltd. All rights reserved.