Treatability studies were performed on oily wastewaters produced by petroleum and canning industries. Chlorine dioxide was used for the removal of phenolic compounds from these oily wastewaters. Most of phenolic compounds can be destroyed by chlorine dioxide within 15 minutes if CI02-to-phenol ratios of higher than 5.0 are provided. Factors such as pH, temperature, and COD have little effect on phenol removal. The effectiveness of chlorine dioxide treatment depends critically on the performance of the chlorine dioxide generator. High yields of chlorine dioxide generation can be achieved by maintaining the pH between 2.5 and 3.5, and by controlling the concentration of feed chemicals. For small treatment plants, chlorine dioxide treatment may be an economical process because no expensive equipment is required. / Master of Science