A procedure of assay of cobalt traces is described, involving the separation of Co using Dowex IX-8 and dithizone extraction. Adsorption and elution phenomena are studied for quantities varying from 0,01 μg to 1 μg of cobalt for which the separation is still quantitative. Errors due to loss in the successive analytical processes, such as evaporation, calcination, mineralisation, extraction, etc., have been determined and are expressed numerically. A radioisotope tracer method is proposed for the determination of the losses.