Ozone pretreatment studies of wastewater containing a specific azo dye – Orange II - were conducted in order to assess the kinetics of ozone oxidation and to evaluate the effect of ozonation on the biodegradability of the wastewater. Batch experiments were performed at different initial concentrations of the dye, showing that ozone is capable of a rapid disruption of the dye molecule. Moreover, the production of biodegradable compounds is apparent from the evolution of COD and BOD5 measurements. A mathematical model which describes the dye elimination, the COD and BOD5 variation, and the amount of ozone reacted has been developed.