The determination consists in co-reduction, with liberation of oxygen, of the peroxyhydrate and potassium permanganate in an acid solution. Because of the specifity of washing powders and the relative stability of peroxyhydrates, the dissolution of washing powders is likely to be incomplete and may result in a loss of oxygen; these disadvantages have been avoited by adoption of a particular method of dissolution appropriate to the determination of active oxygen content. This method may be used for determing peroxyhydrates, for example sodium perborate.