"Ozonation and ultraviolet (UV) irradiation were evaluated for their effects on the treatment of biorefractory organic solutes in an oil shale process wastewater (Oxy-6 retort water). Direct mineralization of dissolved organic carbon (DOC) was determined for each method and for combined UV irradiation/ozonation, but emphasis was placed on the use of these oxidative methods to effect structural alterations, thereby promoting subsequent biooxidation. Short sequential or combined exposures to low dosages of UV and ozone did not affect mineralization of DOC nor did they promote biooxidation of previously biotreated (spent ) retort water. Prolonged exposure to UV did not effect significant photochemical oxidation. In contrast, five hours of a high ozone dosage altered 14 percent of the recalcitrant DOC, thereby promoting subsequent microbial mineralization. Similar results were obtained after only three hours of combined high-dosage UV/ozone treatment; the trend of increasing biodegradability with increasing dosage, however, was reversed after three hours of treatment. Exhaustive biooxidation followed by extensive UV irradiation in combination with ozonation and by secondary biooxidation achieved total elimination of color and odor from Oxy-6 retort water, but more than 41 percent of the DOC was refractory to this series of treatments. Direct chemical and photochemical oxidation and the action of excited oxygen species, ozone decomposition products, and organic radicals are discussed; the ramifications of structural alterations of recalcitrant nitrogen heterocycles are considered."