Thermostable poly(amideimide)s (PAIs) were synthesized according to a ’one-pot/two-step’ direct polycondensation of flexible aromatic diamines with monoesters of isomeric tricarboxylic acid anhydrides containing an aryloxy group or with the monoesters of trimellitic anhydride (TMA). The prepared PAIs have inherent viscosity values of 0.46-0.95 dl g(-1), are soluble in strongly polar aprotic solvents and form tough and transparent films. The molecular structure of the polymers was investigated by Fourier transform infrared and H-1 and C-13 nuclear magnetic resonance spectroscopy. A constitutional disorder due to a random sequence of amide and imide groups was found. Measurements by wide-angle X-ray diffraction indicated that the prepared PAIs are amorphous. The thermal behaviour was studied by differential scanning calorimetry and dynamic thermogravimetric analysis. The glass transition temperatures (T-g) of the PAIs obtained from monoesters of tricarboxylic acid anhydrides containing an aryloxy group range between 262 and 213 degrees C, and are lower than those of the corresponding PAIs derived from TMA. The prepared PAIs have high thermal resistance in nitrogen as shown by their temperatures of 10% of weight loss (T-d), which are higher than 500 degrees C. Both T-g and T-d depend on the orientation of the aryloxy group in the tricarboxylic acid anhydride moiety.