The instant invention is directed to storage stable mixtures in the form of solutions or solution-suspensions comprising:(a) from 10 to 80 percent by weight of aromatic isocyanatosulphonic acids;(b) from 20 to 90 percent by weight of a liquid ester of an inorganic or organic acid of phosphorous, which liquid ester represents a solvent for the aromatic isocyanato sulphonic acids; and(c) from 0 to 70 percent by weight of non-sulphonated organic polyisocyanates;Said percents by weight based on the total weight of (a)+(b)+(c).The mixtures of the instant invention are useful in producing isocyanate addition products.