An in-depth study into the synthesis of the titanosilicate ETS-10 has been carried out to obtain crystals with different particle sizes, roughness and porosity. The effect of these parameters on the CO2/CH4 gas separation performance using mixed matrix membranes (MMMs) has been studied. MMMs based on ETS-10 polycrystalline particles of 1-2 µm in size with high surface roughness and porosity gave rise to a good filler dispersion and filler-polymer interaction. The addition of 10 wt% ETS-10 polycrystalline particles into the polysulfone matrix increased the CO2 permeability from 6.1 to 7.8 Barrer and the CO2/CH4 selectivity from 31 to 38. When using the polyimide 6FDA-6FpDA, a glassy polymer with high gas permeability, the addition of 10 wt% ETS-10 polycrystalline particles increased the CO2 permeability from 96 to 125 Barrer, with a decrease in CO2/CH4 selectivity from 56 to 51.