Monovalent inorganic acids such as hydrofluoric acid can be removed from spent pickling baths by bringing the bath, which shall have a content of sulphuric acid at least equivalent to the metal ion content of the bath, into contact with an organic liquid which contains a compound which forms an adduct with the monovalent inorganic acids. The organic phase extracts the monovalent inorganic acids from the aqueous phase. Metals can then be recovered from the aqueous phase, if desired. The organic phase may then be washed with water, resulting in the monovalent inorganic acids being transferred from the organic phase into the water. The acid aqueous solution thus produced may be used for preparing a fresh pickling bath.