Laser flash photolysis of aqueous acidic solutions of 2-nitrothiophen (NT) over a wide range of acidity has enabled the pKa of the lowest excited triplet state of 2-nitrothiophen (3NT) to be determined as –0.95 ± 0.20. The effect of acidity on the reactivity of 3NT towards a number of one-electron donors has been examined. In particular, it was found that protonated 3NT (3NTH+) oxidizes chloride ion with a second-order rate constant, k2, which is more than an order of magnitude higher than that measured for 3NT. For the quenching of 3NT by inorganic anions (e.g. NO–2, SO2–3) which protonate at lower acidities than 3NT the values of k2 depend on the acid–base properties of the quencher. Finally, the second-order rate constants for the quenching of 3NT by metal ions in acidic media are generally lower than would be predicted on the basis of their thermodynamic potentials.