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 (m ± s) 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 U∑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 U∑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.