In this study, the stoichiometry of decolorization of four dyes (Basic Yellow 28, Malachite Green, Reactive Black 5 and Reactive Red 198) was investigated using ozonation processes. The decolorization of Basic Yellow 28 (BY28) and Malachite Green (MG) was optimal at an acidic pH value of 3, whereas decolorization of Reactive Block 5 (RB5) and Reactive Red 198 (RR 198) was optimal at a basic pH of 10. Stoichiometric Ratios (SR) between ozone and dyes were calculated to be 3.5, 7.1, 11 and 16.1 for BY28, MG, RB5 and RR198, respectively. The correlation between ozone-double bond (DB) with the best linear fit and the least relative error (epsilon (%)) was between aromatic ring number (A(N)(R)) and SR, with regression coefficients of 0.993 and 4.6, respectively. Also, the relationship between total double bond number (DBN(T))-SR had respective values of 5.1 and 0.983 for the epsilon (%) and regression coefficient. Additionally, the pseudo-first order decolorization rate constants, W, were decreased from aromatic ring number 2 (A(2)(R)) to aromatic ring number 5 (A(5)(R)). (C) 2009 Elsevier B.V. All rights reserved.