In this work the preparation and characterization of gas separation hollow fibers based on polyethersulfone Sumikaexcel (PES) and polyimide Matrimid 5218 (PI) blends are reported. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and structure of the asymmetric hollow fibers. Differential Scanning Calorimetry (DSC) was used to determine the glass transition temperatures of the blends. The permeation rates of CO2 and N-2, for both uncoated and PDMS coated fibers, were measured by the variable pressure method. Experimental results were used in conjunction with the resistance model in order to determine the surface porosity of uncoated fibers. PES/PI fibers exhibit an asymmetric structure and one glass transition temperature. Increase of the air gap distance during spinning results in higher surface porosity and gas permeation rates. PDMS coated fibers exhibit a CO2 permeance varying from 31 to 60 GPU and a CO2/N-2 selectivity varying from 35 to 40, at room temperature. The thickness of the skin layer, which corresponds to these permeation rates, varies from 0.1 to 0.15 mum. After plasticization with CO2, ultrathin hollow fibers especially rich in PES, exhibit reversible reduction of CO2/N-2 selectivity. These properties establish PES/PI hollow fibers as excellent candidate membranes for the separation of gaseous mixtures in industrial level.