Electrochemical treatment (EchT) of tumours, which is also known as electrochemical therapy, implies that tumour tissue is treated with a direct current through the use of electrodes. This was shown earlier when the major electrochemical reactions at the anode were chlorine and oxygen evolution and the impact of chlorine in EchT was investigated by mathematical modelling. In this work the electrochemical illustration of a mathematical model of electrochemical treatment of tumours is presented using a Pt electrode placed in agar-agar gel containing 1% phenolphthalein and NaCl. The appearance of the bright red color around the Pt wire due to the pH change caused by hydrogen evolution is used for the simulation of the shape and the boundaries of the tumour tissue. After the polarity was changed, oxygen evolution takes place on the Pt wire electrode and the red color disappears simulating the destructive influence of the electrochemical process on tumour tissue. A better understanding of the mathematical model presented earlier is achieved and it can be used for educational purposes in oncological praxis because agar-agar is the closest approximation to the ground substance of connective human tissue and successfully simulates the processes which occur in vivo. There is also some quantitative agreement between the results calculated earlier and those obtained in this work. The previous model was based on tissue, which was treated as an aqueous solution of sodium chloride containing bicarbonate and proteins and the results presented a detailed and useful quantitative analysis of the system. In this work, the quantitative analysis presented is based on an agar-agar system and the experimental conditions give the possibility of more detailed qualitative analysis. The combination of these two models offers a better simulation of the processes in vivo. (C) 2003 Elsevier Science B.V. All rights reserved.