Aromatic diamines react with pyromellitic dianhydride insolvents such as dimethylacetamide to form solutions of high molecular weight polyamic acids. These soluble polymers can be converted to insoluble aromatic polyimides. The aromatic polyimides exhibit aero strength temperatures above 700 degrees C. and possess excellent thermal and oxidative stability. The stability characteristics of films and the relation of chemical structure to thermal and hydrolytic stability and to polymer properties such as crystallinity are described.