A series of speciation diagrams for nickel species in chloride solutions at 25 degrees C has been plotted with the effects of the ionic strength on the activity coefficient of the hydrogen ions and the equilibrium quotients being taken into account. It has been determined that the predominant nickel species in the acidic region are Ni2+ and NiCl+ in concentrated pure NiCl2 solutions and Ni2+, NiCl+ and NiSO4 in concentrated sulfate-containing NiCl2 solutions. The species NiOH+ is negligible until the NiCl2 concentration is lowered to the order of 10(-3) M. Other nickel species including Ni(OH)(2(aq)), Ni(OH)(3)(-), Ni(OH)(4)(2-), Ni2OH3+ are negligible both in the acidic and basic regions for the concentrated NiCl2 solutions. When pH increases, insoluble Ni(OH)(2(s)) is expected to form in the basic region if NiCl2 concentration is higher than 10(-6) M. The pH where Ni(OH)(2(s)) starts to form decreases with increasing NiCl2 concentration.