A laboratory study was conducted to investigate the ability of Amberlite IRN-78 ion-exchange resin for the removal of nitrate from different waters. Aim of the present study is to propose a simple model that simulates the equilibrium data for this ion-exchange system. Tests were performed using two synthetic waters and two groundwaters. In the case of synthetic water, experimental data are reported for the ion-exchange equilibria determination in the absence or presence of chloride. For both groundwater types, "Salvatierra city" and "Miajadas city", an ion-exchange Langmuir-type model is proposed, taking into account the presence of other competitive anions in solution, specifically chloride, sulphate and bicarbonate. Although the model is very simple, it is consistent with the experimental results. Finally, a general Langmuir-type expression q = (4.36C*)/(C0T+1.58C*) is proposed for all water types, synthetic or natural. This equation considers the presence of other anions in solution by means of the parameter C-0T. (c) 2005 Elsevier B.V. All rights reserved.