A method using nitric acid alone for the digestion of biological materials for the determination of total selenium or of some of its ionic species leads to erroneous results because of incomplete mineralisation of some organic selenium compounds, including the selenonium derivatives, the main metabolites of the element in urine, and a selenoamino acid present in plant and animal tissues; such a method also suffers from many interferences owing to the presence of incompletely digested organic constituents of the sample. The use of perchloric acid under certain conditions and of sulphuric acid, in conjunction with nitric acid, leads to an excellent recovery of the selenium added to biological materials or included in organic selenium compounds. The nitric acid method, the only one which affords a selective determination of some selenium ionic species in aqueous solution, has been critically examined but its efficiency cannot be improved; all of the parameters investigated are helpful, however, in choosing an adequate digestion procedure for the determination of the total selenium content of a complex sample.