An experimental apparatus was constructed which allowed for in situ chemical sampling of a hyperfiltration (reverse osmosis) system inside the high-pressure reservoir. In this system - a columnar apparatus 50.6 cm in length and 5.1 cm in diameter- a 101.3 mM sodium chloride solution was advected towards an uncompressed sodium-saturated bentonite membrane. Twelve small-diameter stainless steel tubes were fitted to run parallel to the length of the experimental column-each tube terminating at a different position within the concentration polarization layer (CPL). Solution extracted from these different positions within the CPL showed a progressive buildup of solute toward the membrane's high-pressure interface. This measured concentration profile agrees favorably with the geometry of the CPL as predicted by solution of the continuity equation. These measurements constitute the first direct verification of the CPUs geometry under conditions where the fluid flow is directed toward the membrane and there is no cross-flow component. (c) 2006 Elsevier B.V. All rights reserved.