Herein, a series of novel triptycene-containing thermally rearranged poly(benzoxazole-co-imide) membranes (TPI-PBOs) with enhanced mechanical properties and gas separation performance are reported. The effects of chemical structures and poly(benzoxazole-co-imide) composition on the fractional free volume, polymer chain packing, microcavity size and size distribution, mechanical properties, and gas transport properties have been comprehensively investigated. Due to the incorporation of triptycene-containing non-TR-able polyimide segments, the resulting poly(benzoxazole-co-imide) films exhibited good mechanical properties even after being treated at 450 degrees C. The incorporation of bulky and rigid triptycene units led to significantly improved fractional free volume and gas transport properties compared to previously reported poly(benzoxazole-co-imide)s. Several triptycene-containing poly(benzoxazole-co-imide) films displayed excellent gas separation performance for H-2/CH4, H-2/N-2, and CO2/CH4 gas pairs that exceeded the 2008 upper bounds.