Effects of dissolved oxygen concentrations on dye removal by zero-valent iron (Fe-0) were investigated. The Vibrio fischeri light inhibition test was employed to evaluate toxicity of decolorized solution. Three dyes, Acid Orange 7 (AO7, monoazo), Reactive Red 120 (RR120, diazo), and Acid Blue 9 (AB9, triphenylmethane), were selected as model dyes. The dye concentration and Fe-0 dose used were 100 mg L-1 and 30g L-1, respectively. Under anoxic condition, the order for dye decolorization was AO7 > RR120 > AB9. An increase in the dissolved oxygen concentrations enhanced decolorization and chemical oxygen demand (COD) removal of the three dyes. An increase in gas flow rates also improved dye and COD removals by Fe-0. At dissolved oxygen of 6 mg L-1, more than 99% of each dye was decolorized within 12 min and high COD removals were obtained (97% for AO7, 87% for RR120, and 93% for AB9). The toxicity of decolorized dye solutions was low (l(5) < 40%). An increase in DO concentrations obviously reduced the toxicity. When DO above 2 mg L-1 was applied, low iron ion concentration (13.6 mg L-1) was obtained in the decolorized AO7 solution. (c) 2010 Elsevier B.V. All rights reserved.