DFT calculations have been performed to investigate the effect of several different substituents on the N-O bond dissociation enthalpies of 2-substituted-quinoxaline-1,4-dioxides. Further, the influence of electron and proton affinities and also of N-O bond dissociation enthalpies on the antitumor activity of selected compounds is analyzed. In agreement with experimental data, the calculations suggest that Tirapazamine is more active than Quindoxin due to a much easier reduction and protonation of the former species accompanied by a weaker N-OH bond. (c) 2006 Elsevier B.V. All rights reserved.