The stepwise formation constants of the kinetically labile iodomercurates(II) in acetonitrile are relatively high; even for the tetraiodo complex a value of 1220 was determined. Ligand-to-metal charge transfer transitions of different energies and intensities feature the ultraviolet spectra of these compounds. Photolysis of their CH3CN solution results in the formation of Hg(l) and I or I2− as primary products. The quantum yields of this reaction are 0.8 and 0.41 for HgI3− and HgI42−, respectively, at 355 nm excitation in a deaerated system. Due to the very efficient back reaction HgI2 does not undergo permanent photoredox change, while irradiation of tri- and tetraiodomercurate(II) complexes leads to the accumulation of I2 and I3−. The efficiencies of these overall reactions are rather low (0.07 and 0.02 at 333 nm), compared to the primary quantum yields, because of the fast reconversion.