Polydimethylsiloxane (PDMS) has been extensively studied as a separation membrane due to the high permeability of gases and organic solvents. Here we report 60-nm-thick PDMS membranes prepared by interfacial crosslinking of PDMS solution using N-2-plasma. It was found that the presence of hydrocarbon in PDMS accelerates the crosslinking reaction, giving mechanically tough ultrathin membranes with swellable pores of 4 to 5 nm. Molecular modelling calculation suggested that dissociation of carbon-silicon bonds is the major pathway of crosslinking reaction.