This comparative study focuses on the cyclodehydration reaction of poly(hydroxyamide)s (PHAs) and the thermal rearrangement (TR) reaction of aromatic poly(hydroxyimide)s (APIs) to produce poly-benzoxazoles (PBOs) for application as gas separation membranes. Systematic comparisons between the two routes showed that PHA cyclodehydration can produce fully converted PBO membranes at a temperature more than 100 degrees C lower than the API-TR route. Gas separation performance of PHA-PBOs is comparable to API-TR polymers, and PHA-PBOs are considerably more mechanically robust. More importantly, PBO films can be produced equally well in both air and inert atmospheres via the PHA cyclodehydration route. The PHA-PBOs formed in air have statistically the same physical and transport properties as those formed in N-2. The present study provides fundamental information regarding the PHA-to-PBO cyclodehydration process. (C) 2015 Elsevier Ltd. All rights reserved.