Abstract: The method for preparing cross-linked vinylaromatic polymers containing nitro groups having the general formula ##STR1## where R' is H, NO.sub.2, COOH, SO.sub.3 H, P(O)(OH).sub.2, or P(O)H(OH), m is from 60 to 99 and n is from 40 to 1, consists in treatment of the starting cross-linked polymers having the general formula ##STR2## where R is H, COOH, SO.sub.3 H, P(O)(OH).sub.2, or P(O)(OH), m is from 60 to 99 and n is from 40 to 1, with a mixture of an alkali metal nitrate and an acid agent, at a temperature from -5.degree. to 95.degree. C. The quantity of the alkali metal nitrate in the nitrating mixture is from 0.1 to 8 moles per mole of the starting cross-linked polymer.The method provides conditions for preparing cross-linked vinylaromatic polymers, containing nitro groups, having higher mechanical strength, high osmotic stability, and their volume in an ion-exchange column changes insignificantly as acid media are changed for alkaline, and vice versa.