The excited triplet state of 1-nitronaphthalene (³1NN*) reacts with OH⁻ with a second-order reaction rate constant of (1.66 ± 0.08)×10⁷ M⁻¹s⁻¹ (μ ± σ). The reaction yields the ˙OH radical and the radical anion 1NN⁻˙. In aerated solution, the radical 1NN⁻˙ would react with O₂ to finally produce H₂O₂ upon hydroperoxide/superoxide disproportionation. The photolysis of H₂O₂ is another potential source of ˙OH, but such a pathway would be a minor one in circumneutral (pH 6.5) or in basic solution ([OH⁻] = 0.3–0.5 M). The oxidation of H₂O by ³1NN*, with rate constant 3.8 ± 0.3 M⁻¹s⁻¹, could be the main ˙OH source at pH 6.5.