Novel aromatic polymers containing a nitrogen-containing ring group which comprises at least 30 mol% of a recurring unit of the formula ##STR1## Preferably,, the nitrogen-containing ring group ##STR2## is a benzimidazolone, benzimidazolthione, benzimidazole, quinoxalidinedione, benzotriazole, benzothiazolidine, benzothiadiazolidine-S-oxide, benzothiadiazolidine-S-dioxide, benzodithiadiazine, benzodiazophosphole-p-oxide, benzodiazophosphole, benzodiazosilole, or benzodiazoborole ring.These polymers are soluble in various polar solvents. They find a wide range of applications ascribable to these ring groups, for example, as thermally stable resins and permselective membranes. These polymers can be prepared by reacting substantially linear aromatic polymers comprising at least 30 mol% of a recurring unit of the formula ##STR3## with reagents capable of forming 5- or 6-membered nitrogen-containing rings.