The aqueous sulfite ion reacts with Hg2+ ((aq)) to form 1:1 and 2:1 coordination complexes. The Hg(SO3)(2)(2-) complex is redox stable. However, dissociation of a sulfite ligand forms redox unstable HgSO3. Under conditions where Hg(SO3)(2)(2-) predominates, the rate of reduction of the mercuric ion to Hg-0 by coordinated sulfite depends inversely on the concentration of uncoordinated sulfite, while it is unaffected by the amount of sulfite liberated by dissociation; Analysis of the kinetics yields the sequential sulfite binding constants K-1 = 2.1 x( 10(13) and K-2 = 1.0 x 10(10) at mu = 0.10 M. These values lead to the prediction that HgSO3 is more abundant in clouds than is Hg(SO3)(2)(2-) under virtually all atmospheric conditions. The product of the redox reaction appears to be a strongly bound Hg-0. SO2 complex, which is at least 3 orders of magnitude more soluble than uncomplexed Hg-(aq)(0). This finding may have important implications for the partitioning of atmospheric mercury from the gas phase into atmospheric water droplets prior to its wet deposition.