The leaching of oxides in high strength brines is advantageous because of favourable leaching kinetics even at atmospheric conditions due to enhanced proton activity, and the catalytic effect of chloride ions due to complexation with metal ions. A proper understanding of hydration theory, practice of measuring proton activities in concentrated acid-chloride systems, and their applications to rationalise leaching kinetics is beneficial for further development of chloride processes. In the present study, a hydration number of I or 2 for the chloride ion and a constant ionic activity coefficient for the hydrated chloride ion has been proposed for the theoretical evaluation of pH of concentrated hydrochloric acid solutions. A comparison of the results based on hydration theory and extended Debye-Huckel equation with the experimental results based on acidity functions, glass electrode, and hydrogen electrode shows the importance of liquid junction potential corrections for measured values based on potentiometric methods. The proton activity based on electrode potentials reveals a hydration number of 6-7 which is consistent with an ion-pair hydration model (H3O+)(H2O)(6-7)Cl-. Chloride ions facilitate the dissolution of copper(II) oxide and goethite in acid solutions. The rates of dissolution of these oxides in hydrochloric acid solutions are first order with respect to proton activity due to the formation of adsorption complexes such as Cu(OH)Cl-0 and Fe(OH)(2)Cl-0. Hydration, chloro-complexation and hydrolysis of metal ions play important roles in the selective leaching of nickel over iron from laterite ores by acid-chloride lixiviant system. (C) 2007 Elsevier Ltd. All rights reserved.