Object of the invention are new diisatoic acid and triisatoic acid derivatives of the general formula I ##STR1## wherein n represents 2 or 3, X stands for oxygen or sulfur and R for a group of the general formula IIY--Z--B].sub.m, (II)in which m stands for 1 or 2, Y for B when m is 1 and Y further represents a di- or trivalent alkyl group with 1 to 5 carbon atoms when m is 2, B represents alkylene with 1 to 4 carbon atoms and Z stands for phenylene-1,4-dioxy or diphenylene-4,4'-dioxy optionally singly or multiply substituted by Cl, NO.sub.2 or alkyl or for a 5- to 7-membered heterocyclic ring which is optionally substituted by one or more of said substituents and has one or more atoms selected from the group consisting of N, S and O as members of the ring, with a benzene nucleus optionally being annulated to said ring, and a process for the production of these compounds which is characterized in that a compound of the general formula IIIR--XH).sub.n (III)wherein R, X and n have the meaning defined above is reacted with at least two equivalents of isatoic acid anhydride, preferably in the presence of strong bases, as well as the use of these new compounds for the preparation of plastic materials.