Ordered mesoporous silica spheres (MCM-41) were used as fillers in polyimide (Matrimid) based mixed-matrix membranes (MMMs). The filler particles were functionalized with sulfonic acid (–SO3H) groups to increase the separation performance of the membranes by increasing the CO2 solubility. The fast diffusion of gases through the mesoporous materials, accompanied by this increased interaction with CO2, resulted in the simultaneous increase of gas permeability and selectivity. The functionalized membranes showed up to 31% increase in CO2 permeability and 14% increase in CO2/CH4 selectivity. In order to study the stability of these newly developed MMMs, they were tested at different operating conditions (temperatures, mixed-gas feeds and CO2 feed concentration). •MMMs based on Matrimid and –SO3H functionalized.•Good dispersion of the functionalized fillers in the polymer matrix.•Significant increase in permeability and selectivities was obtained.