Aqueous hydrogen sulfite reacts with Hg2+ to form, in the absence of excess HSO3-, the HgSO3 complex, observed here for the first time. Its UV spectrum is described by epsilon(234 nm) = (1.57 +/- 0.05) x 10(4) M-1 cm(-1). HgSO3 decomposes in an intramolecular redox reaction which is kinetically first-order. The rate constant is independent of [pHg(2+)], [HSO3-], [O-2(aq)], and ionic strength. An acid-assisted pathway becomes significant at pH less than or equal to 1, attributed to the contribution of HgSO3H+. The rate of the intramolecular reaction of HgSO3 was measured by trapping the Hg-0 product as Hg-2(2+); the value of the rate constant is k(0) (0.0106 +/-0.0009) s(-1) at 25.0 degrees C, pH 3. The activation parameters for pH 3,Delta H-double dagger and Delta S-double dagger, are (105 +/- 2) kJ/mol and (68 +/- 6) J/mol.K, respectively, consistent with a unimolecular bond cleavage mechanism. A pathway involving H2O-induced concerted 2e(-) transfer is proposed.