The air separation through triheptyl cellulose (THC) /ethyl cellulose (EC) blend membranes containing no more than 20 wt % THC at the temperature range from 298 to 358 K was investigated using a variable volume method. The air-separation ability for the THC/EC membranes were greater than that for the THC-free pure EC membrane. P(O2) for the THC/EC membranes was between 1.06-8.89 X 10(-9) cm3 (STP) cm/cm2 s cmHg and P(O2)/P(N2) 3.04-3.66. The THC/EC membrane showed a unique trend in its P(O2)/P(N2)-P(O2) relationship, i.e., the magnitude of P(O2)/P(N2) increased simultaneously with that of P(O2). The THC/EC membrane yielded a maximum oxygen concentration in the oxygen-enriched air (OEA) of 39.5% at an OEA flux of 6.99 X 10(-4) cm3 (STP) / s cm2 for a pressure difference of 0.43 MPa at 358 K. After 300 h of measurement at 0.40 MPa and 313 K, the efficiency of the concentrating oxygen was almost constant.