A technique for separation of Mg, with 100% yield from low-Mg biogenic carbonates has been developed suitable for high precision analysis of Mg isotopes by multiple-collector inductively coupled plasma mass spectrometry. Two separate stages of ion-exchange chromatography were carried out using cation exchange resin, AG50W-X12 with a Mg recovery >99.9%. The repeatability of the Mg isotope ratio measurement using this technique including chemistry and mass spectrometry is ±0.14‰ on δ²⁶Mg and ±0.09‰ on δ²⁵Mg at 95% confidence. This was demonstrated using a synthetic solution (Na∶Mg∶Ca∶Sr = 2∶1∶100∶1 in weight) over a period of six months. The robustness of the technique was further assessed by replicate analyses of three natural samples, seawater, foraminifera and dolomite. A total variation over 4.5‰ on δ²⁶Mg was observed and the Mg isotope composition of seawater was 2.59 ± 0.14‰ on δ²⁶Mg and 1.33 ± 0.09‰ on δ²⁵Mg which were the highest isotope ratios among the samples measured.