It has long been recognized that the performance of supported membranes comprising a thin, selective layer and a microporous support layer, is affected by penetration of the selective layer into the porous support. We have attempted to shed light on this phenomenon using a combination of pervaporation experiments and hard angle dark-field scanning transmission electron microscopy (HAADF-STEM). We use a nanophase-separated polystyrene-b-polydimethylsiloxane-b-polystyrene (SDS) as the selective layer and microporous polytetrafluoroethylene (PTFE) as the support layer. Effective permeabilities of butanol and water were measured as a function of selective layer thickness using a dilute butanol/water mixture as the feed in pervaporation. We were able to estimate the pore penetration layer thickness by comparing experiments with model calculations. We were also able to directly observe the pore penetration by HAADF-STEM. The choice in using a nanophase-separated block copolymer as the selective layer enabled identification of the regions of pore penetration. The pore penetration layer thickness obtained from the HAADF-STEM micrographs corresponded well with estimates based on pervaporation.