In this paper, we present a critical path analysis (CPA) of transport through a "selective surface flow" membrane operating in a hydrogen purification application. Using experimental mixture permeability data as an input, the CPA allows us to estimate the structural parameters of the membrane. In addition, the CPA gives insight into the role of network connectivity, showing that the species selectively transported through the membrane (methane, in this case) and the species which the membrane is intended not to transport (hydrogen) pass through essentially distinct sub-networks within the pore network of the membrane. (C) 2003 Elsevier Ltd. All rights reserved.