Cancer cells are more sensitive to oxidative stress due to higher levels of reactive oxygen species. Therefore, the ability of anti-cancer agent combretastatin A-4 (CA-4) and triazole analogues to induce reactive oxygen species may be important for selectivity against cancer cells. The purpose of the present study was to investigate the structural requirements for reactive oxygen species production by CA-4 and the triazole analogues Ana-2, Ana-3 and Ana-4. Ana-2 and Ana-3 mimic the cis configuration in CA-4; Ana-3 lacks the phenolic hydroxyl group, while Ana-4 mimics a trans configuration. The rat pheochromocytoma cancer cell line PC12 was used as model system. CA-4 and Ana-2 were highly toxic; Ana-3 was less toxic, whereas Ana-4 was non-toxic. The probe dihydroethidium detected reactive oxygen species production from CA-4, Ana-2, and Ana-3. CA-4 and Ana-2 also induced oxidation of the reactive oxygen species probe dihydrorhodamine and activation of caspase-3. Thus, the phenolic hydroxyl group in CA-4 and Ana-2 was necessary for dihydrorhodamine oxidation, caspase-3 activation, and increased cytotoxicity.