A series of novel melt-processable copolyimides based on mixed thioetherdiphthalic anhydride (TDPA) isomers and 4,4'-oxydianiline (4,4'-ODA) were synthesized by a one-step polycondensation. Molecular weight of copolyimides was controlled with stoichiometric offsets in favor of 4,4'-ODA. All of the resulting polyimides (PIs) were amorphous and have nice solubility. With increase in molecular weight, the glass transition temperatures of copolyimides increased from 248 to ca. 270 A degrees C based on DSC measurements. These copolyimides displayed excellent thermal and thermooxidative stability, as evidenced by thermogravimetric analysis in both air and nitrogen atmospheres. The melt processability, thermoplastic response, melt stability, and their dependence on molecular weight of the copolyimides have been investigated in terms of rheological measurements by monitoring the melt viscosity after residence at various times and temperatures.