This article concerns the synthesis and characterization of novel tricomponent amphiphilic membranes consisting of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic polydimethylsiloxane (PDMS) segments cocrosslinked and reinforced by octasilane polyhedral oligomeric silsesquioxane (octasilane-POSS) cages. Rapid and efficient network synthesis was effected by cocrosslinking diallyl-telechelic PEG (APEG-A) and divinyl-telechelic PDMS (V-PDMS-V) with pentamethylpentacyclosiloxane (D5H) using Karstedt's catalyst in conjunction with Et3N cocatalyst and water. Films were prepared by pouring charges in molds and crosslinking by heating at 60 degreesC for several hours. The films were characterized by Sol fractions and equilibrium swelling both in hexane and water, extent of crosslinking, contact angle hysteresis, oxygen permeability, thermogravimetric analysis, and mechanical properties. The crosslinking of octasilane-POSS achieved by the same catalyst system was studied in separate experiments. (C) 2004 Wiley Periodicals, Inc.