Methanesulfonyl iodide is produced in aqueous solutions from the reaction of triiodide with methanesulfinate. Dichroic crystals of (CH3SO2I)(4)center dot KI3 center dot 2I(2), are formed from KI/I-2 solutions with high concentrations of CH3SO2-, while dichroic crystals of (CH3SO2I)(2)center dot RbI3 are formed from RbI/I-2 solutions. X-ray crystallography of these two compounds shows that the CH3SO2I molecules coordinate through their oxygen atoms to the metal cations and that the S-I bond length is 2.44 angstrom. At low concentrations of CH3SO2-, the solutions remain homogeneous and the sulfonyl iodide is formed in a rapid equilibrium: CH3SO2- + I-3(-) (sic) CH3SO2I + 2(I-), K-MSI = 1.07 +/- 0.01 M at 25 degrees C (mu = 0.1 M, NaClO4). The sulfonyl iodide solutions display an absorbance maximum at 309 nm with a molar absorptivity of 667 M-1 cm(-1). Stopped-flow studies reveal that the equilibrium is established within the dead time of the instrument (similar to 2 ms). Solutions of CH3SO2I decompose slowly to form the sulfonate: CH3SO21 + H2O -> CH3SO3- I- + 2H(+), k(hyd). In dilute phosphate buffer, this decomposition occurs with k(hyd) = 2.0 x 10(-4) s(-1); the decomposition rate shows an inverse-squared dependence on [I-] because of the K-MSI equilibrium.