||The structure of triphenylselenonium chloride dihydrate has been determined by single-crystal X-ray diffraction techniques. Crystals of the compound C18H15Se+, Cl−, 2H2O are monoclinic, space groupC2/c, witha = 12.798(3),b = 13.802(1),c = 19.933(34) Å, β = 9.0.0(1) ° andZ = 8. The structure was refined by full-matrix least-squares methods from 1444 diffraotometer intensities to a conventionalR of 0.067 and a weightedR of 0.072. The structure consists of pairs of triphenylselenonium cations which are weakly joined through pairs of bridging chloride ions. The water molecules are clustered in groups of four, separated by the triphenylselenonium chloride dimers. The hydrogen atoms of the water could not be positioned unambiguously, but it appears that hydrogen bonds link the four water molecules and the chloride ions together.