A method is described for the separation and determination of cobalt in 100 mg reactor-grade zirconium. Separation is effected by means of an ion-exchange resin. After elution, the cobalt is purified by extraction with dithizone and determined spectrophotometrically with the help of nitroso-R-salt. With this method a content of 0.0003% can be determined with an accuracy of ±4%.