We report an effective example of n-conjugated systems showing the reverse spin-state preferences depending on the substitution patterns. It is shown that 2,3,7,8- and 1,4,6,9-tetramethoxy-substituted thianthrenes have different electronic structures owing to destabilization of the specific frontier molecular orbitals. On the basis of the hybrid Hartree-Fock/density functional method (B3LYP/6-31G*), the singlet-triplet energy gaps are estimated to be -10.7 and +8.8 kcal/mol for 2,3,7,8- and 1,4,6,9-substituted thianthrenes, respectively. Moreover, it is also shown that nuclear independent chemical shift (NICS), the index of local aromaticity for polycyclic pi-conjugated systems, is closely correlated with the spin-state preferences of the present thianthrene system.