Ozonation of the commercially important, recalcitrant reactive dye intermediate 2-naphthylamine 3,6,8-trisulphonic acid (K-Acid) was investigated. Ozonation performance was examined by following ozone absorption rates and K-Acid, chemical oxygen demand and total organic carbon removals. Mean oxidation states and unidentified organic products were also determined. At pH 3, where direct ozone reactions are dominant, the second-order rate constant between K-Acid and molecular ozone was determined as 20 M-1 s(-1) for steady-state aqueous ozone concentration. The competition kinetics approach was also adopted where a reference compound, phenol, and K-Acid were subjected to ozonation. By applying this method, the second-order reaction rate constant was found to be 76 M-1 s(-1). Common oxidation products formed during ozonation at pH 3, pH 7 and pH 7 with 1 mM hydrogen peroxide were identified as methoxy-phenyl-oxime, phenol, benzene, benzaldehyde and oxalic acid via high-performance liquid chromatography and gas chromatography/mass spectrometry analyses. Continuous nitrate and sulphate evolution were observed during K-Acid ozonation as a consequence of the abrupt release and subsequent oxidation of its amino and sulphonate groups. The number and amount of reaction products were most intensive for K-Acid ozonation at pH 7 with 1 mM hydrogen peroxide. According to the acute toxicity tests conducted with Vibrio fischeri, ozonation products were not less toxic than the original K-Acid solution that caused only 15% inhibition.