Mg2+ and Ca2+ at concentrations typically found in hard waters (30-100 mg L-1) promote THM formation in the disinfection process by hypochlorite. This THM enhancement occurs for model solutions as well as for natural water (Turia river). The use of model solutions has allowed establishing that THM promotion is a rather general effect for polyols, citric and humic acids, while for some other substrates such as histidine and dicarboxylic acids the influence of the presence of Mg2+ and Ca2+ is negligible. The results with Me2+ and Ca2+ have been compared with that achieved with Cu2+. It has been observed that the effect of THM enhancement follows the order Cu2+ > Mg2+ > Ca2+. (C) 2009 Elsevier B.V. All rights reserved.