Separation of magnesium isotopes was investigated by chemical ion exchange with synthesized 1-aza-15-crown-5 bonded Merrifield peptide resin using elution chromatography. The capacity of novel monoazacrown ion exchanger was 2.3 meq g(-1) dry resin. The heavier isotopes of magnesium are concentrated in the resin phase, while the lighter isotopes are enriched in the solution phase. The glass ion-exchange column used in our experiment was 35-cm long with an inner diameter of 0.2 cm, and 2.0 M NH4Cl solution was used as an eluent. The single-stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of Mg-24(2+)-M-25(2+), Mg-24(2+)-Mg-26(2+), and Mg-25(2+)-Mg-26(2+) were 1.014, 1.026, and 1.012, respectively.