Electrochemical treatment of tumours implies that tumour tissue is treated with a direct current. During electrolysis, electrical energy is converted to chemical energy through electrochemical reactions at the electrodes. The anode is preferably placed in the tumour and the cathode in a blood vessel or in fresh surrounding tissue. The main electrochemical reactions are chlorine and oxygen evolution, at the anode? if platinum is used. Hydrogen evolution takes place at the cathode. The aim of this paper is to show how mathematical modelling can be used as a tool for defining and optimising the operating conditions of electrochemical treatment (ET) of tumours. A simplified mathematical model is presented for direct current treatment of tumours, focusing on tissue surrounding a spherical platinum anode. The tissue is treated as an aqueous solution of sodium chloride and only the major electrochemical reactions are considered. The model is based on transport equations of ionic species in dilute solutions. Kinetic expressions for the electrochemical reactions, at the anode surface, are introduced. Inputs to the model are the applied current density, and sizes of the anode and electrolyte domain. Concentration profiles of the ionic species and potential distribution, as a function of time. are calculated. In addition, current yields of the anode reactions are obtained from the model.