The chloro-nickel(II) complexes obtained in dimethyl sulphoxide (DMSO) are described, viz., the octahedral complexes [NiCl(DMSO)5]⁺ and [NiCl2(DMSO)4]⁰ and the tetrahedral complexes [NiCl3(DMSO)]^â and NiCl4 ^2â. It is shown that the tetrahedral species produced on warming a solution of NiCl2 is [NiCl3(DMSO)]^â, not NiCl4 ^2â as previously claimed. The effect of temperature upon the various equilibria existing in solution at different Cl : Ni mole ratios is discussed and the effect of cation variation is noted. For the equilibrium [NiCl2(DMSO)4]⁰+ Cl^â â [NiCl3(DMSO)]^â+ 3DMSO, Î H was calculated to be +13·3 ± 0·3 kcal. mole^{â 1}.