Wastewaters produced in textile industrial processes contain organic dyes, such as azo dyes, which are not amenable to direct biological treatment. This work examines the possibility of combining ozonation with biological treatment for the effective treatment of wastewaters containing the azo dye Orange II. Oxalate, formate, and benzene-sulfonate are found to be the final products of ozonation. Sulfonated aromatics such as benzene-sulfonate cause organic pollution and are not readily degradable because of their xenobiotic character. For the biological treatment of the ozonation products, activated sludge acclimated to the main product (benzene-sulfonate) was used, and batch experiments under aerobic conditions were performed. On the basis of experimental data, mathematical models describing both processes were developed. An overall mathematical model describing the integrated chemical and biological treatment of the dye was used to estimate the cost of the whole process. The implications of this study for the design of effective dye-containing wastewater treatment plants are discussed.