Ibuprofen, a frequently detected pharmaceutical in natural and engineered waters, was studied in both neutral and anionic forms using density functional theory at the B3LYP/6-311++G**//B3LYP/6-31G* level of theory in its reaction with hydroxyl radical (•OH). The reaction pathways included •OH addition to aromatic ring, abstraction of a H-atom, and nucleophilic attack on the carbonyl group. The results showed that H-atom abstraction pathways are the most favorable. The free energy change for H-atom abstraction reaction ranges from −37.8 to −15.9 kcal/mol; for •OH addition ranges from −3.85 to −1.23 kcal/mol; and for nucleophilic attack on the carbonyl group is 13.9 kcal/mol. The calculated rate constant between neutral ibuprofen and •OH, 6.72 × 109 M−1s−1, is consistent with the experimental value, 6.5 ± 0.2 × 109 M−1s−1. Our results provide direct evidence for byproduct formation and identification on the molecular level. © 2013 Wiley Periodicals, Inc.