Breakthrough mode liquid chromatography was employed to study calcium isotope fractionation. Highly porous silica beads, the inner pores of which were embedded with a benzo-18-crown-6 ether resin or a benzo-15-crown-5 ether resin, were used as column packing material. For both the resins, enrichment of heavier isotopes of calcium was observed in the frontal part of their respective calcium chromatograms. The values of the isotope fractionation coefficient were on the order of 10 73 and 10 74for the benzo-18-crown-6 ether and benzo-15-crown-5 ether resins, respectively. The observed isotope fractionation coefficient was dependent on the concentration of hydrochloric acid in the calcium feed solution; a higher hydrochloric acid concentration resulted in a smaller fractionation coefficient value. The present calcium isotope effects were most probably mass dependent, indicating they came from isotope effects based on molecular vibration. Molecular orbital calculations supported the present experimental results in a qualitative fashion.