The purification of cesium by ion exchange with a synthetic aluminosilicate zeolite was demonstrated on a laboratory scale for use in preparing exceptionally pure137Cs irradiation sources. Cesium ions are preferentially loaded on a column of granular Zeolon from a solution that contains Na+, K+, and Rb+impurities. The impurities are eluted from the column with a dilute ammonium carbonate scrub solution which also elutes a small amount of the cesium during the latter part of the scrub cycle. To minimize the cesium waste loss, the latter portion of the scrub is recycled to the column ahead of the next batch of feed. The cesium remaining on the Zeolon column is eluted with a concentrated ammonium carbonate solution. The ammonium carbonate eluant is evaporated to yield a cesium carbonate which contains<1% total metal ion impurities. Cesium waste losses were also<1%.