Morphology and gas separation properties of composites based on ExtemA (R) polyetherimide containing finely divided filler have been analyzed in a wide concentration range. Diamond blend having a high surface area with different functional groups has been selected as the fine filler. Physicochemical, structural, and gas transport characteristics of film samples obtained by dry and dry-wet spinning have been determined. It has been suggested that the introduction of the fine carbon filler in a concentration of 4 to 5 wt % causes structural changes, which result in an abrupt change in the gas transport properties.