Direct and indirect (sensitized) photolysis of p-tert-octylphenol (OP), 4-octylphenoxy-acetic acid (OP1EC), and ibuprofen (IBU) were investigated in laboratory water and surface water under simulated and natural sunlight conditions. Photodegradation obeyed apparent-first order kinetics with rates increasing in the presence of NO₂⁻, NO₃⁻, and humic acid (HAC). The bimolecular rate constants, k″, were determined for the reactions of OP and OP1EC with hydroxyl radical (·OH) using photolyzed hydrogen peroxide (H₂O₂) as the hydroxyl radical (·OH) and IBU as the reference compound. The k″ values for OP and OP1EC were (average and standard deviation) (10.9 ± 0.5) × 10⁹ M⁻¹ s⁻¹ and (8.6 ± 0.5) × 10⁹ M⁻¹ s⁻¹, respectively. Direct photolysis of OP is small with a quantum yield of 0.015 in the range of 285-295 nm. Based on laboratory and average solar intensity data, the estimated half-life of OP in different Singapore surface waters was estimated to range from 0.6 to 2.5d. The steady state hydroxyl radical concentration ([·OH](ss)) was estimated using a kinetic model that considered dissolved organic carbon compounds (DOC), nitrate, and nitrite as ·OH sources, and DOC, CO₃²⁻ and HCO₃⁻ as scavengers. In surface waters containing DOC 2.3-6.5 mg L⁻¹, nitrate 0-3.2 mg L⁻¹, and nitrite 0-2.5 mg L⁻¹, the calculated [·OH](ss) ranged from 5.2 × 10⁻¹⁵ to 9.6 × 10⁻¹⁵ M. Half-lives calculations based on this model underestimated the measured half-life by a factor of approximately 4.2 to 1.1. DOC was predicted to be the most important sensitizer except in a sample that contained relatively high nitrate and nitrite. In the presence of NO₃⁻, photoreactions produced nitrated OP and IBU. A mechanism for OP photolysis in the presence of nitrate is proposed.