Calcium ions were made to electromigrate along a cation exchange membrane. The abundance ratios of the calcium isotopes (Ca-40, 42, 43, 44, 48) in the migrated bands were measured by thermo-ionization mass spectrometry. The lighter isotopes were enriched in the front part of the migrated band. The increments in the isotope abundance ratios were found to be proportional to the mass differences of the isotopes. The observed e-values per unit mass difference (ε/ΔM) were 1.26 x 10 -4 (at 20 °C), 1.85 x 10 -4 (at 25 °C) and 2.4 x 10 -4 (at 40 °C). The mass spectrometry was improved by using a low temperature for the evaporation of CaI 2 .