An aqueous solution of bisulphite blocked polyisocyanate prepolymers is imparted improved storage stability at temperatures up to about 45.degree. C by incorporation of about 0.5 to 20% by weight (based on the weight of bisulphite blocked prepolymer) of an aromatic or alkyl aromatic sulphonic acid of the formula:R(SO.sub.3 H).sub.nwherein R is an aromatic or alkyl aromatic hydrocarbon group having 6 to 15 carbon atoms and n is an integer from 1 to 3. The solution may also contain up to 80 volume percent (based on the total quantity of solvent) of an organic solvent miscible with water and having a boiling point below about 150.degree.C. This solution may be used to finish paper, leather or textiles and to shrink proof keratin containing textiles.