Synthesis of high-flux composite membranes requires deposition of a selective, ultrathin skin on a highly permeable support. This paper describes the use of room-temperature atom transfer radical polymerization (ATRP) from a substrate to synthesize two types of ultrathin (similar to50nm) polymer skins: cross-linked poly(ethylene glycol dimethacrylate) (PEGDMA) and linear poly (2-hydroxyethyl methacrylate) (PHEMA). The synthetic procedure involves attachment of an initiating layer to modified porous alumina and subsequent polymerization from the initiator. This method is attractive because film thickness can be easily controlled by varying polymerization time, and field-emission scanning electron microscopy (FESEM) images indicate that grafted polymer skins can completely cover a porous surface without filling underlying pores. Gas permeation studies show that PEGDMA has a CO2/CH4 selectivity of similar to20 and an O-2/N-2 selectivity of similar to2; whereas uncross-linked PHEMA films show very little selectivity. However, the selectivity of PHEMA improves significantly after derivatization with fluorinated acid chlorides. The selectivities of these systems demonstrate that ATRP is indeed capable of forming ultrathin, defect-free membrane skins that can potentially be derivatized for specific separations. (C) 2003 Elsevier B.V. All rights reserved.