DILUTED CAUSTIC (NAOH, 5-10 WT %) SOLUTIONS ARE COMMONLY USED IN THE OIL REFINING INDUSTRY FOR THE DESULFURIZATION OF HYDROCARBON STREAMS [1].DURING CAUSTIC SCRUBBING, GASEOUS SULFUR COMPOUNDS SUCH AS H2S AND THIOLS ARE ABSORBED RESULTING IN WASTE STREAMS KNOWN AS SULFIDIC SPENT CAUSTIC. WITHOUT TREATMENT, THESE STREAMS MAY IMPOSE ENVIRONMENTAL PROBLEMS BECAUSE OF THEIR ALKALINITY (PH>12), SALINITY ([SODIUM] OF 5-12 % WT) AND HIGH SULFIDE (HS- AND S2-) LEVELS EXCEEDING 2-3 WT % [2].IN THIS STUDY, PERFORMANCE OF ADVANCED OXIDATION PROCESS (UV/H2O2 OXIDATION SYSTEM) FOR TREATING SULFIDIC SPENT CAUSTIC FROM TABRIZ PETROCHEMICAL COMPLEX WAS INVESTIGATED. THIS TECHNIQUE CAN BE AN ALTERNATIVE METHOD TO WAO THAT THE CURRENT SYSTEM IS WORKING IF THE RESULTS BE ACCEPTABLE.EXPERIMENTS WERE PERFORMED IN CYLINDRICAL STAINLESS STEEL REACTOR WHICH IS FIXED WHIT A LOW PRESSURE MERCURY VAPOR LAMP IN THE AXIAL POSITION.PERFORMANCE MEASUREMENT SYSTEM WAS EVALUATED DAILY, TOTAL SULFIDE, COD AND PH.IN UV SYSTEM, SULFIDE ALMOST COMPLETELY CONVERTED TO SULFATE AND THIOSULFATE. THIS CAUSES THE COD DECREASES AND THEN INCREASES AGAIN. H2O2 DOES NOT HAVE ABILITY TO OXIDIZE THE SULFIDE.IN UV/H2O2 SYSTEM, WITH THE INCREASE OF H2O2 DOSAGE TO AN OPTIMUM VALUE, REMOVAL EFFICIENCIES OF COD WERE INCREASED AND A BETTER PERFORMANCE WAS OBTAINED THAN THE H2O2 SYSTEM ALONE. IN UV/H2O2 SYSTEM, REMOVAL OF EFFICIENCY OF COD REACHED 68% UNDER THE OPTIMUM CONDITION.