The excited triplet state of 1-nitronaphthalene, 1NN, (³1NN) is able to oxidise nitrite to ˙NO₂, with a second-order rate constant that varies from (3.56 ± 0.11) × 10⁸ M⁻¹ s⁻¹ (μ ± σ) at pH 2.0 to (3.36 ± 0.28) × 10⁹ M⁻¹ s⁻¹ at pH 6.5. The polychromatic quantum yield of ˙NO₂ photogeneration by 1NN in neutral solution is Φ_˙NO2^1NN ≥ (5.7 ± 1.5) × 10⁷ × [NO₂⁻]/{(3.4 ± 0.3) × 10⁹ × [NO₂⁻] + 6.0 × 10⁵} in the wavelength interval of 300–440 nm. Irradiated 1NN is also able to produce ˙OH, with a polychromatic quantum yield Φ_˙OH^1NN = (3.42 ± 0.42) × 10⁻⁴. In the presence of 1NN and NO₂⁻/HNO₂ under irradiation, excited 1NN (probably its triplet state) would react with ˙NO₂ to yield two dinitronaphthalene isomers, 15DNN and 18DNN. The photonitration of 1NN is maximum around pH 3.5. At higher pH the formation rate of ˙NO₂ by photolysis of NO₂⁻/HNO₂ would be lower, because the photolysis of nitrite is less efficient than that of HNO₂. At lower pH, the reaction between ³1NN and ˙NO₂ is probably replaced by other processes (involving e.g.³1NN-H⁺) that do not yield the dinitronaphthalenes.