Integrally skinned asymmetric gas separation membranes of polyethersulfone (PES)/polyurethane (PU) blend were prepared using supercritical CO

2

(SC-CO

2

) as a nonsolvent for the polymer solution. The membrane consisted of a dense and a porous layer

which were conjoined to separate CO

2

from CH

4

. The FTIR

DSC

tensile and SEM tests were performed to study and characterize the membranes. The results revealed that an increase in SC-CO

2

temperature causes an increment in permeance and a decrease in membrane selectivity. Furthermore

by raising the pressure

both permeance and selectivity increased. The modified membrane with SC-CO

2

had much higher selectivity

about 5.5 times superior to the non-modified membrane. This higher selectivity performance compared to previous works was obtained by taking the advantages of both using partial miscible blend polymer due to the strong polar-polar interaction between PU PES and SC-CO

2

to fabricate the membrane. The response surface methodology (RSM) was applied to find the relationships between several explanatory v

ariables and CO

2

and CH

4

permeance and CO

2

/CH

4

selectivity as responses. Finally

the results were validated with the experimental data

which the model results were in good agreement with the available experimental data.