The thermal and thermo-oxidative stabilities of aromatic polyimides have long been recognized. Most of the polyimides were synthesized by condensation of a dianhydride (PMDA), the properties of polyimide films are greatly influenced by diamine component. The polymer chains derived from p-phenylene diamine (PPD) and benzidine (BZD) are more rigid and their films are brittle, but they have better thermal stability. The polymer chains derived from 4,4'-diamino-diphenyl ether (APE) and 4,4'-diaminodiphenyl sulfide (ADS) are flexible and can be cast to flexible films, but these films are not as thermally stable as the above mentioned. In most applications, the thermal stability and flexibility need to reach some extent of compromise. In this paper, we are reporting the preparation and properties of two copolyimides made by reaction of PMDA with two diamines PPD/ADS and BPDA (3,3',4,4'-biphenyl tetracarboxylic dianhydride) with PPD/ADS. Synthesis of thio containing copolyimides gave polymers with a high degree of thermal stability and enough flexibility to be useful in many actual applications.