The imidization temperature of poly(amide-acid) compositions derived from tetracarboxylic acid dianhydrides can be lowered by use of a bis(aminophenoxyphenyl) phosphine oxide in the diamine component which is reacted with the dianhydride. These homo- or copolyimides can be cyclized without use of catalysts or excessive temperature which can product undesired side reactions. Tough film forming materials can be produced which show excellent thermo-oxidative stability and high char yields even at 800.degree. C. in air.