The CO2 gas and water vapor transport properties of a novel aliphatic polyamide with an ethyl branch were investigated. The polymer was characterized with density measurements, differential scanning calorimetry, thermogravimetric analysis, and wide-angle X-ray diffraction analyses, and the amorphous and glassy nature of the polymer at the ambient temperature were confirmed. The CO2 sorption isotherm of the polymer appeared to obey the dual-mode sorption isotherm, which was characteristic of the glassy state. The water vapor sorption below a relative humidity of 0.4 or 0.5 was explained in terms of the Brunauer-Emmett-Teller sorption mechanism, whereas that at a high relative humidity demonstrated a dissolution type of water vapor into the polyamide. The permeability coefficients of He, CO2, O-2, and N-2 gases through the membrane were as follows: P(He) > P(CO2) > P(O-2) > P(N-2). The novel polyamide membrane was more permeable to CO2, O-2, and N-2 gases than nylon 6 and nylon 66 membranes, containing a crystalline and hydrogen-bonding nature. (c) 2005 Wiley Periodicals, Inc.