The concept of S

T-isomers was first introduced by Polansky and Zander. Since then

the concept has been extended and many properties of such isomers have been studied. Recently many results of the S

T-isomers were obtained and the monographs have been published[7]

[8]. Most of the results were obtained by comparing the numbers of aromatic x-sextets and the numbers of Kekule structures of S isomers and T isomers.In this paper a new type of S

T-isomers is examined. They are Benzenoid systems formed from four subunits A

A′

B and a central one C

where B is symmetry and A' is isomorphic to A. It is proved that an S-isomer does not have more aromatic π-sextets than its corresponding T- isomers and in some occasion the number of Kekule structures of an S-isomer never exceeds that of its. corresponding T-isomer

while in some other occasion this is not so.