A deep desulfurization process for light oil has been investigated, on the basis of photosensitized oxidation using a triplet photosensitizer and hydrogen peroxide in an oil/water two-phase Liquid-liquid extraction system. Dibenzothiophene (DET) in tetradecane was effectively desulfurized. This was achieved via the indirect photoexcitation of DBT, using benzophenone (BZP) under photoirradiation conditions of wavelength lambda > 280 nm, and the effective oxidation of the excited DBT by H2O2, suppressing the photodecomposition of H2O2. The desulfurization of commercial light oil was also achieved in this process by the addition of both BZP and H2O2, and the sulfur content was reduced from 0.2 to 0.05 wt % by 48 h of photoirradiation, to meet with the new regulation in Japan. Although C5 and C6 alkyl-substituted DBTs were found to be the most difficult components to be desulfurized, they too were desulfurized effectively in the presence of BZP and H2O2. The photodecomposed sulfur-containing compounds removed into the water phase were separated successfully, using column adsorption with aluminum oxide as the adsorbent. The H2O2 solution recovered is also reusable for the desulfurization process.