Potassium has to be determined accurately in studies of geological age by the potassium-argon method. This paper recommends a procedure for the accurate determination of potassium. In order to overcome the interferences from other elements in rocks and minerals, potassium was separated from aliquots of sample solutions by cation-exchange chromatography on Dowex 50W × 4 or 71-616 resin columns Solutions were prepared gravimetrically instead of volu- metrically. Possible contamination due to potassium in glassware was avoided by the use of platinum, quartz and polyethIend vessels. The accuracy may be greatly raised after the determination of potassium by AAS. Six standard samples were measured many times in this way, and the re- sups agreed well with those obtained from laboratories in U. S. A. and Austra- lia. The relative standard deviations do not exceed 3% of the mean result within 1-10% of potassium. The procedure may be applied to the accurate determination of potassium in K-Ar dating and other fields.