Montmorillonite is one of the most commonly used 2:1 phyllosilicates. A characteristic feature of the smectites is a high degree of structural disorder leading to a high dispersity of the cation exchange capacity (CEC). The electrostatic interactions due to the CEC are important in a lot of industrial processes. We examine the electrostatic properties of Otay type montmorillonite as a function of the octahedral configuration. We have chosen two models of cis-vacant sites and two models of trans-vacant sites. A plane-wave density functional theory has been used to optimize the atomic positions and perform a Mulliken population analysis. The results of the hydrogen position and the electrostatic charges of the different layers are presented. From these results, the trans-vacant configuration shows a higher hydrophilic character than the cis-vacant configuration. (C) 2004 Elsevier B.V. All rights reserved.