AbstractSynthesis of methoxy poly(ethylene glycol) (PEG) functionalized polyhedral oligomeric silsesquioxane (POSS) nanoparticles via epoxy ring opening reaction in three different solvents are outlined in this manuscript. The nanoparticles are used as filler for commercial poly(ether-block-amide) multiblock copolymer PEBAX® MH 1657. The influence of two novel structural features of the synthesized nanofillers on the gas separation performance of nanocomposite membranes are studied on the examples of CO2/N2 and CO2/H2 gas pairs. These are – (i) presence of a dimethylsilyl group as spacer between the cage structure of POSS and the PEG ligand and (ii) formation of a tetrahydrofuran (THF) complex. While ideal selectivity characteristics for the matrix polymer are not significantly affected by the presence of fillers, the single gas permeability (determined by time-lag method) is remarkably increased in both cases.