This work reports on the chemical isotope fractionation of Zn(II) by a solvent extraction method with the crown ether dicyclohexano-18-crown-6. The Zn-m/Zn-64 ratios (m = 66, 67, and 68) were analyzed by multiple-collector inductively coupled plasma mass spectrometry. The relative deviations of the Zn-66/Zn-64 ratios relative to the unprocessed material (delta Zn-66) was determined to be -0.51 to -0.32 in the acidity region 1.0-6.0 mol dm(-3) (M) HCl. The acidity dependence or delta Zn-m was explained by the isotope exchange reactions between Zn(II) species (Zn2+, ZnCl+, ZnCl2, ZnCl3-, and ZnCl42-) and the mole fractions of them. The magnitude of delta Zn-m due to the related Zn(II) species estimated by quantum chemical calculations was in agreement with delta Zn-m experimentally obtained. Contribution of nuclear field shift to the isotope fractionation was estimated to be less than 10% of delta Zn-m by quantum chemical calculations.