Electrochemical treatment (EChT) of tumours offers considerable promise as a safe, simple and relatively noninvasive antitumour therapy. When platinum is used as electrode material, the major electrochemical reactions at the anode are chlorine and oxygen evolution. Conflicting opinions can be found in the literature over the role of chlorine in the underlying destruction mechanism behind EChT. In this present study, the impact of chlorine in EChT treatments is investigated by means of mathematical modelling. The analysis focuses on the tissue surrounding a spherical platinum anode when applying a constant current density. Tissue is modelled as an aqueous solution of sodium chloride containing a bicarbonate buffer system and organic constituents susceptible to reactions with chlorine. Except for the case of very low anode current densities, the simulations clearly show that it is the spreading of hydrogen ions - and not chlorine molecules - that determines the extent of tissue destruction around the anode. Moreover, it is found that the reactions of chlorine with tissue play important roles as generators of hydrogen ions. The contribution of these reactions to the acidification of tissue, surrounding the anode, is strongly dependent on the applied current density and increases with decreasing current density.