Simulated reactive dyebath effluent (pH11.0-11.5) bearing the commercially important textile azo dye Reactive Red 21 (100 mg/L) can be successfully decolorized via ozone (feed rate=72 mg/min) and peroxide (=0.75-6.00 mM)/ozone treatment processes. Persulfate (PS), peracetic acid (PAA) and percarbonate (PC) were selected as alternative oxidants to the more conventional hydrogen peroxide. Color (peak absorbance) and total organic carbon (TOC) removals increased with increasing PS concentration, but decreased when PC was introduced due to free radical scavenging effects of carbonate alkalinity. PAA improved color and TOC removal rates, but thereby also contributed to the TOC content of the dyebath effluent. PAA-assisted ozonation showed the highest performance in terms of color removal at an optimum PAA concentration (=1.5 mM). According to the gas phase-and-dissolved ozone measurements, addition of the peroxides PS-PAA enhanced ozone decomposition and increased ozone absorption rates whereas PC addition stabilized aqueous, molecular ozone. The originally non-toxic reactive dyebath effluent (10% relative inhibition) did not exhibit serious acute toxicity towards Vibrio fischeri photobacterium throughout ozonation and ozone/PS treatment, whereas a slight increase was observed for ozone/PAA-ozone/PC that decreased with applied ozone dose. The 7th-day-biochemical-oxygen-demand (BOD7; <10 mg/L) decreased with ozone dose speaking for less biodegradable oxidation products.