Titanium(IV) oxide (TiO2) nanoparticles have been incorporated into mixed matrix membranes (MMMs) to improve gas separation performance. However, TiO2 nanoparticles tend to agglomerate due to high surface energy and van der Waals forces. This leads to precipitation which causes the formation of non-homogeneous MMM morphology. In this study, the effect of octaisobutyl polyhedral oligomeric silsesquioxane (POSS) addition on TiO2/polysulfone MMM was investigated. The aims are to enhance gas separation performance whilst preventing agglomeration of TiO2 nanoparticles. The results demonstrated that inclusion of POSS as dispersant increases MMMs� CO2/CH4 selectivity and permeance, possibly due to less void formation and more evenly distributed pore structure. For example, synergistic addition of 5 wt TiO2 and 5 wt POSS increased the CO2/CH4 selectivity up to 390 compared to MMM without POSS. This is supported by elemental mapping of titanium which revealed that POSS successfully dispersed TiO2 nanoparticles and prevented aggregation. TiO2-POSS/PSf MMMs also retained their favorable thermal stability. © 2019, Chinese Chemical Society Institute of Chemistry, Chinese Academy of Sciences Springer-Verlag GmbH Germany, part of Springer Nature.