The gas transport properties of the mixed matrix membranes (MMM) prepared from the synthesized, structurally different poly(hydroxyimide)s (HPI) and mesoporous MCM-41 silica particles were investigated. The both series of filled and unfilled precursors were used to obtain and examine thermally rearranged (TR) polybenzoxazoles (PBO). The effect of a number and position of hydroxyl groups in HPI chains as well as that of filler presence on TR conversion was studied using FTIR, DSC, TGA, WAXD, SEM, mechanical testing, and density measurements. With 7 wt% of MCM-41 silica, gas permeability of the filled HPI precursors and that of the respective filled PBOs increased 1.5-2.2 and 4.4-15.1 times, respectively, comparing to the neat HPIs. The permeability increase along with relatively low loss of gas selectivity for filled membranes shift them towards the upper bound demonstrating that incorporation of porous fillers into polybenzoxazoles precursors can be a new approach to improve membrane performance.