The presence of boron compounds in waters increases in a continuous and parallel way to industrial development. Therefore, their harmful effects on living organisms also increases, especially on plants, since this element manifests an important micronutrient-toxic boron duality. The aim of this study is to investigate the influence of different operation variables within the adsorption process of the boron compounds with magnesium oxide when liquid waste of urban, agricultural or industrial origin is being treated. The results obtained indicate that the process is strongly influenced by the quality of added reagent and by the contact time between the reagent and solution. Moreover, the temperature variable also stands out, as it has a very positive influence, reducing the necessary contact time to obtain specific boron removal yields. On the one hand, it has been observed that this process appears to be inextricably linked to pH. The removal process improves as the pH increases, presenting a maximum at pH value between 9.5 and 10.5, which is where borate ion predominates. The reagent used in the study has an important alkalinising capacity. Due to the fact that the pH of the solutions is situated around this range, it is not necessary to adjust this variable during the process. Therefore, using this reagent is an attractive option. As for the metallic cations that usually accompany boron in industrial waste, it seems that the reagent's presence also facilitates the removal of these cations and a simultaneous treatment of industrial waste can thus be carried out. Furthermore, interrelationships between the different variables have been established. When the optimum conditions are selected, the process reaches over 95% of boron removal. (C) 2005 Elsevier B.V. All rights reserved.