A series of isomeric poly(thioether imide)s (PTIs) containing thioether linkages were prepared by aromatic nucleophilic substitution reaction of isomeric N-(4-mercaptophenyl)-3-chlorophthalimide and/or N-(4-mercaptophenyl)-4-chlorophthalimide. The inherent viscosities of these polymers were in the range of 0.38-0.42 dL/g. The glass transition temperature increased with increasing the content of 3-substituted phthalimide unit in the polymer backbone. The 5 % weight loss temperatures of isomeric PTIs (a-e) were 506-531 A degrees C in nitrogen and 509-536 A degrees C in air, respectively. The isomeric PTIs (b-d) films exhibited good mechanical properties with tensile strength of about 90 MPa, elongation at break of more than 6.0 %, and tensile moduli of more than 2.2 GPa. The storage moduli of PTIs (b-d) almost completely maintained at about 2.0 GPa before reaching the corresponding glass transition temperatures. It is noted that the minimum melt viscosity of these polymers decreased with increasing the content of unsymmetrical 3- and 4-substituted phthalimide unit.