To reduce the boron concentration in desalinated water by reverse osmosis (RO) the most used technology is the second RO step. The membranes employed are typical of brackish water working at high pH. In this article, the kinetic parameters of transport through the membranes were determined and the influence of pH was studied. A pilot plant fed with desalinated seawater was used. Boron in water is mainly in the form of boric acid in equilibrium with borate. The variations in pH alter the relative proportion of these species. The differences observed in the transport of boron as a function of the pH do not occur solely due to the physical properties of the membrane, but are the result of two interactions: the species present of boron, boric acid borate, and the characteristics of the membrane. The coefficient of reflection of boron and of the permeability of the species has been calculated. The influence of pH on the boric acid-borate equilibrium justifies the marked differences observed in the rejection of boron at high and low pH, while the influence of pH on the values of permeability coefficients and reflection of each species justifies the differences observed between membranes. (C) 2010 Elsevier B.V. All rights reserved.