In this work the fabrication of new mixed-matrix membranes (MMMs) of Matrimid (R) and activated carbon (AC) for gas separation is reported. The aim is the fabrication of membranes that have better gas permeation properties compared to the pristine Matrimid (R) membranes. The membranes were thermally and morphologically characterized, and the gas transport properties of single gases were estimated by a variety of methods. It has been found that with an increase of the AC content the selectivity remained stable for the different gases despite the marked increase in the effective permeability of the pure gases.