Thermostable aromatic polyimides have been prepared from mellitic trianhydride and aromatic diamines. The effect of certain variables on the homogeneous solution polycondensation, and consequently obtained intermediate polyamic acids were investigated. Aromatic diamines employed were p-phenylenediamine, 4, 4'-diaminodiphenyl ether and 4, 4'-diaminodiphenylmethane reaction. These polyamic acids were converted to the polymellitimides by heat cyclization. Polyimides cast in the form of thin films were flexible, insoluble and infusible. A comparison of thermogravimetric analyses and infrared spectroscopic results between obtained polymellitimides was also made together with a brief discussion of their thermostability.