Abstract Nitroso-R-salt method has been investigated for the purpose of determining a minute quantity of cobalt in minerals. In some previous papers it was reported that the method is very satisfactory for determining cobalt in plants, soils and animal nutritions. In this study it has been proved thát the method is also fairly satisfactory for determining cobalt in minerals containing a large amount of iron. Cobalt content of some arsenopyrites has been determined by this colorimetric or the gravimetric method. The result shows that all the samples contain a minute or fairly large quantity of cobalt. However, the problem in what form cobalt is contained in these samples remains for the subsequent studies.