The shortage of drinking water is a major problem in the South East of Spain. In these areas, it is essential to make use of water from underground reservoirs, most of which are over exploited and suffer from saline contamination given their proximity to the sea. The desalination of brackish water is a means of obtaining low cost drinking water. Electrodialysis is a technique based in the transport of ions through selective membranes under the influence of an electrical field. This technique has proved its feasibility and high performance in the desalination of brackish water, the desalting of amino acids and other organic solutions, effluent treatment and or recycling of industrial process streams and salt production. In this paper, a mathematical model for the desalination of brackish water through controlled potential electrodialysis has been developed. The application of this model allows: (i) to predict the behaviour of the system, (ii) to calculate the electrical energy consumption and (iii) to calculate the necessary time for successful desalination. The model has been applied with satisfactory results to the desalination of a NaCl solution in different experimental conditions. The model developed could be applied to commercial electrodialyzers (pilot/industrial plant) working in batches with recirculation and controlled potential, which is the usual mode of operation of such equipment when the requirements for treated water are moderated.