The transport properties of gases in PPO (poly(2,6-dimethyl-1,4-phenylene oxide))-silica mixed matrix membranes (MMMs) were improved using a thermal annealing method. The density and crystallinity properties of the MMMs were characterized using TGA, XRD, FESEM, and FTIR to clarify the predominant mechanism of thermal annealing. The results indicate that the thermal-annealing treatment can induce the polymer chains to re-crystallize by annealing above T-g rather than causing them to exhibit solvent vaporization when MMMs were used. After thermal annealing, a significant enhancement in the gas separation, thermal stability, and plasticization resistance was observed for the PPO-silica MMMs. PPO-S5-220 (5 wt.% silica) MMM produced the highest H-2 permeability of 257.23 Barrer and the highest H-2/CO2 selectivity of 3.6. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.