Commercially available polysulfone membranes are known to be hydrophobic and are prone to fouling due to strong interaction with variety of solutes. In order to overcome this. the 5 K molecular weight cut off (MWCO) and 20 K MWCO polysulfone ultrafiltration membranes have been modified by gas phase nitration using NOx (a mixture of NO and NO2). These membranes were subsequently aminated using hydrazine hydrate and as a result of this, membranes were observed to give considerably higher rejection for nonionic molecules (polyethylene glycol) with simultaneous increase in resistance to the flow. Experimental measurements have shown that the modified membranes have the exchange capacity of 4.17 meq per dry g of membrane and we show that this exchange capacity can be obtained only when there are about two exchangeable amine groups per repeat unit of every polysulfone molecule. A careful examination of the increase in resistance suggests that one of the mechanisms of separation of nonionic solutes is due to the reduction of average pore size of the membranes, However, for sodium chloride solution we show that the modified membranes have a 13%, rejection (as opposed to none for unmodified one), while for a complex feed, like alkaline black liquor. the rejection (,For all dilutions) is twice that for unmodified membranes. Due to this modification, it appears that the membrane is entering into nanofiltration zone.