Phase inversion method is applied to prepare flat sheet asymmetric polymeric membranes from homogeneous solution of 12 wt% polysulfone (PSf) with two different solvents-N-methyl-2-pyrrolidone (NMP) and dimethyl acetamide (DMAc). 5.0 wt% polyethylene glycol (PEG) of three different molecular weight (400, 6000, and 20,000 Da) is used as the polymeric additives in the casting solution. Membranes are characterized by two different techniques viz. scanning electron microscopy (SEM) and gas permeation tests. Finally, the results of both the techniques are compared with those calculated from pure water permeation tests using Hagen-Poiseuille equation. It is found that though the values obtained from all the techniques vary from each other, their trend with increase in molecular weight of PEG seems to be the same. It is seen that when molecular weight of PEG increases from 400 to 20,000 Da, the mean pore size of the prepared membranes decreases, while the porosity and pore density show an increasing trend; the pressure normalized gas flux rises significantly and the thickness of the top layer of the prepared membrane sheet increases. (C) 2008 Elsevier Inc. All rights reserved.