Ozonation was effective to degrade sulfaquinoxaline: more than 99% of efficiency was obtained using an ozone dose of 2.8 mg L-1 and pH 3. A design of experiments with two variables, pH and ozone dose, was used for planning. There was no significant interaction between these variables and ozone doses played the most important role regarding the contaminant degradation. However, regarding toxicity ozone process at pH 3 did not inhibit the luminescence of the bacteria even though different intermediates were formed and identified by mass spectra. At pH 7 inhibition of luminescence remained almost constant (around 30%) throughout ozonation time.