Two series of aromatic poly(ortho-hydroxyamide)s (poly(o-hydroxyamide)s, HPAs) were prepared by reaction of two diamines, 2,2-bis(3-amino-4-hydroxyphenyl) propane (APA) and 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (APAF), with four aromatic diacid chlorides; terephthaloyl dichloride (TPC), isophthaloyl dichloride (IPC), 2,2-bis(4-chlorocarbonylphenyl)hexafluoropropane (6FC) and 4,4'-sulfonyldibenzoyl dichloride (DBSC). Amorphous HPAs with high molecular weights (inherent viscosities higher than 0.5 dL/g) and relatively high glass transition temperatures (220-280 degrees C) were obtained. Dense membranes of HPAs were able to undergo a thermal rearrangement (TR) process to polybenzoxazoles (beta-TR-PBOs) heating at moderate temperatures (between 250 and 375 degrees C), and their complete conversion was reached at a temperature below 375 degrees C, depending on the o-hydroxy diamine moiety, APA and APAF. The B-TR-PBOs films derived from APAF showed a higher thermal stability and higher Tg than those from APA. Gas separation properties of TR-PBOs membranes were superior to those of their poly(o-hydroxyamide) precursors, particularly for the following gas pairs: O-2/N-2, CO2/CH4, He/CH4 and He/CO2.