Blend membranes of 2,2'-bipyridine (2BP) or 4,4'-bipyridine (4BP) with ethyl cellulose (EC) containing no more than 25 wt % BP are prepared and evaluated for their oxygen enrichment by both the constant pressure-variable volume method and the constant volume-variable pressure method. The actual air-separation ability through the 2BP/EC blend membrane containing 1.5-7 wt % 2BP are enhanced while the permeated flux is slightly increased in comparison with the virgin EC membrane. Among the 2BP/EC blend membranes examined, the 2BP/EC blend membrane containing 3 wt % 2BP offers the best oxygen/nitrogen permselectivity and yields the highest oxygen concentration of 42.7% at the transmembrane pressure difference of 0.75 MPa and 25degreesC. Like other homogeneous dense membranes, the BP/EC blend membrane demonstrates strong dependencies on the transmembrane pressure difference, retentate/permeate flux ratio, and operating temperature. It possesses higher activation energy of oxygen and nitrogen permeation than those of the virgin EC membrane in the tested temperature range of 9.7-60degreesC. The CO2 over CH4 permselectivity through EC membranes can be improved by introducing 4BP, and the ideal oxygen over nitrogen separation factor through the 4BP/EC (10/90) membrane increases 16% at the upstream pressure of 10 bar compared with the virgin EC membrane.