Water solutions of organic dye Direct Red 79 were treated by DC high voltage created by diaphragm discharge (input power up to 160 W). Higher dye decoloration (60% after 40 min) was observed in the anode part of the reactor by UV-VIS spectroscopy. Recorded absorption spectra showed shifts of absorption maximum to lower wavelength values that were related to the formation of stable degradation products. Absorption spectra obtained in the cathode part of the diaphragm reactor produced only negligible shifts of absorption maximum and the dye decomposition rate reached 25% after 80 min of the treatment. HPLC-MS method revealed five main compounds in the anode part and another three ones in the cathode part. Analysis of identified products showed on the initial decomposition of one azo-group in the original dye molecule. Significant changes of solution properties during the diaphragm discharge were observed, too. Solution conductivity increased in both parts of the reactor. Solution pH dropped from the initial neutral value to acidic conditions (up to 3) in the anode part and to alkaline conditions (up to 10) in the cathode part. Changes were induced by formation of reactive species by the discharge and by electrochemical reactions on electrodes. (C) 2010 Elsevier B.V. All rights reserved.