NEW MATERIAL: A compound represented by the formula I (wherein: R1 is H, a 1-4C alkyl group, or R2; R2 is a 2-8C monohydroxyalkyl or polyhydroxyalkyl group; R3 is H, a 1-4C alkyl group, or R2; R4 is H or a 1-4C alkyl group; and (n) is 1 or 2).

EXAMPLE: Hydroxymalonic acid-bis(3,5-bis(2,3-dihydroxy-propylcarbamoyl)-2,4,6- triiodoanilide).

USE: X-ray contrast media.

PREPARATION: Such a compound represented by the formula I is prepared by allowing a substituted dicarboxylic acid derivative represented by the formula II (R5 is a 1-4C alkyl or benzyl group, or the like; and Z is a reactive acid or ester group) to react with a base represented by the formula III (R1' is H, a 1-4C alkyl group, or the like; and R2' is a free or protected, 2-8C monohydroxyalkyl or polyhydroxyalkyl group).