Abstract The equilibrium (association) constant, KA, for the first protonation of the sulfate ion in aqueous solution: \([\text{H}^{+}(\text{aq}) + \text{SO}^{2−}_{4}(\text{aq}) ⇌ \text{HSO}^{−}_{4}(\text{aq})]\) has been measured at 298.15 K in NaCl and NaClO₄ media using glass-electrode potentiometry at five ionic strengths in the range 0.5 ≤ I/(mol·dm−3) ≤ 5.0. The present KA values vary smoothly with I and are in good agreement with reliable literature data but indicate that the previously reported value of KA at I = 5 mol·dm−3 in NaClO₄ media is in error.