The complex formation between zinc and chloride ions has been investigated by emfmeasurements on the cell Ag,AgCl 1 .x M NaCl + y M N&IO, t 0.025 M ZnCI, IZn(Hg),Pt. The total concentration was varied between 1 and 4.6mol-I-'. Three mononuclear complexes with I, 3 and 4 chloride ligands were found to be present. The complex constants based on the concentrations of the unhydrated species +dc)-showed a pronounced increase with decreasing water activity. A set of concentration independent constants is obtained when the solution is treated as an ideal mixture of hydrated ions and free water. The number of water molecules in the hydration shell replaced by one chloride ligand was estimated to four and the complex constants to: P,(x) = Bl(c)'(Cc,).&l = 45 P3(X) = &(c).(&)"x& = 4.5 X 104 Ir,(x) = S,(c).(&)~. X& = 4.5 X 105 Where 2~; is the total concentration of hydrated ions and free water, and xH,,, is the mole fraction of free water. NOTATION activity of the unhydrated ion activity of the hydrated ion activity coefficient of the unhydrated ion concentration (mol~l-l) total concentration of the specie i mole fraction of the hydrated ion on the basis of hydrated species mole fraction of free water complex stability constant based on the activities of the ions complex constants based on the molar concentrations of the unhydrated ions complex constants based on the mole fractions of the hydrated ions number of chloride ligands at the zinc ion hydration number of the ion i emf of the Zn (Hg) electrode measured against Ag/ AgCl in the same solution (VI standard mf WI emf obtained by extrapolation to ci = 1 in the actual supporting electrolyte 09