Magnesium chloride was extracted into an organic phase including dicyclohexano-18-crown-6 (DC18C6). The partition coefficient showed its highest value for a liquid-liquid extraction system of 3.5 M MgCl2 aqueous phase. Magnesium isotopes Mg-24, Mg-25, and Mg-26 were enriched in the system with their respective enrichment factors. The enrichment factor for Mg-24/Mg-26 was epsilon = 0.0112 as a maximum value. The enrichment factors for a unit difference of mass numbers varied depending on the MgCl2 concentrations in the aqueous phase. The isotope-recognizing ability of DC18C6 was K-c = 1.0016, which is smaller than the separation factor due to an exchange of the aqua/dichloro complex of Mg2+, K-p = 1.0040. An odd/even isotope effect was observed, and it was assigned to nuclear properties other than mass difference. Odd/even staggering associated with the optical transition of electrons offers a consistent explanation to the different behaviors of isotopes with odd or even mass numbers.