The effects of nitrogen and air purge during thermal rearrangement of an ortho-functional polyamide (o-PA) and an ortho-functional polyimide (o-PI) towards a polybenzoxazole (PBO) structure have been investigated in terms of physicochemical changes and gas transport properties. The o-PA polymer was prepared from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluropropane (BisAPAF) and 4,4'-biphenyl-dicarbonyl chloride (BPDC) while the o-PI polymer was derived from 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3'-dihydroxybenzidine (HAB). Experimental results show that the purge environment for the conditions used does not affect the thermal rearrangement of the o-PA film but significantly affects the thermal conversion of the o-PI film. Nearly identical chemical structures and pure gas permeability values are observed for o-PA films thermally treated at 300 degrees C under air or N-2. These properties become different in the o-PA films treated at 425 degrees C, which is presumed to be attributed to the influence of oxygen on the thermal stability of the derived PBO and probably the various degrees of thermal crosslinking reaction induced at a high temperature. The o-PI film was thermally rearranged at 425 degrees C because its thermal conversion takes place at a higher temperature range of 300 degrees C-450 degrees C. The o-PI film thermally rearranged in air exhibits improved gas permeation properties but significantly deteriorated mechanical properties. The air purge interrupts the thermal conversion of the ortho-functional imide to benzoxazole by oxidatively degrading the imide structure and forming the imine structure. As a result, both polymer structure and film properties change. (C) 2013 Elsevier Ltd. All rights reserved.