The photodegradation of chlorine-based disinfectants NH₂Cl, HOCl, and OCl^- under UV irradiation from low- and medium-pressure Hg lamps was studied. The quantum yields of aqueous chlorine and chloramine under 254 nm (low pressure UV) irradiation were greater than 1.2 mol Es ^-1 for free chlorine in the pH range of 4 to 10 and 0.4 mol Es ^-1 for monochloramine at pH 9. Quantum yields for medium pressure (200-350 nm) ranged from 1.2 to 1.7 mol Es^-1 at neutral and basic pH to 3.7 mol Es^-1 at pH 4 for free chlorine, and 0.8 mol Es^-1 for monochloramine. Hydroxyl radicals (·OH) are a direct photolysis product of both aqueous free chlorine and hydrogen peroxide under UV irradiation. The photooxidation rates of an ·OH probe compound, nitrobenzene, were compared in aqueous solutions of free chlorine, and H₂O₂, exposed to monochromatic UV irradiation (254 nm). Acidic (< pH 6) free chlorine solutions had the highest observed rates of nitrobenzene decay. In deionized water, with initial oxidant concentrations < 0.15 mM, the first order nitrobenzene decay rates increased in the following order of treatments: UV-Cl₂(pH 7) < UV-H₂O₂ < UV-Cl₂ (pH 6) < UV-Cl₂ (pH 5). Four tri alkyl phosphate esters were selected for comparison of their second-order rates of reaction with UV and O₃ synthesized ·OH, in water. In competition with nitrobenzene in UV irradiated hydrogen peroxide solutions (lab grade, deionized water), tris(2-butoxyethyl) phosphate (TBEP) was fastest to react with photosensitized ·OH (k_OH,TBEP=1.03x10 ¹⁰ M^-1s^-1), followed sequentially by tributyl phosphate (TBP), tris(2-chloroethyl) phosphate (TCEP), and tris(2-chloro isopropyl) phosphate (TCPP) (k_OH,TBP=6.40x10 ⁹, k_OH,TCEP=5.60x10⁸, & k_OH,TCPP=1.98x10⁸ M^-1s ^-1). Biodegradable organic carbon (BDOC) from indirect photolysis (UV-H₂O₂) of the recalcitrant industrial anti-foaming agents and flame retardants, tributyl phosphate (TBP) and tris(2-chloroethyl) phosphate (TCEP), was quantified with respect to the fraction of the TBP or TCEP photooxidized. For 50-96% contaminant oxidation via ·OH, BDOC was similar in solutions of either compound, and ranged from 0.25 to 0.5 mg/L (TBP₀ and TCEP₀ = 5 mg/L). In addition, for this contaminant oxidation range, complete dehalogenation of TCEP was observed, along with a significant change in pH. Oxidation of TCEP results in both H⁺ and Cl^- release, while the TBP mineralization pathway results in CO₂, H₂O, and PO₄, only.