Octaanionic meso-tetra(3,5-dicarboxylatophenyl) porphyrin 1 was adsorbed to gold electrodes at pH 12 and stayed there after repeated washing with 10(-2) M KOH. The fluorescence on sputtered gold surfaces amounted to 10% of the intensity observed on an organic subphase. Addition of 10(-6) M aqueous solutions of the manganese(III) complexes of an isomer mixture of tetracationic beta-tetraethyl-beta'-tetrakis (1-methyl-4-pyridinium)- and meso-4-(1-methyl-4-pyridinium)phenyl porphyrins 2 and 4 at pH 12 quenched the fluorescence quantitatively. Visible spectroscopy proved that the amount of porphyrin 1 on the gold surface had not changed. The octaanionic porphyrin 1 was then embedded in a membrane by self-assembly of a bolaamphiphile containing two secondary amide groups. Two hydrogen bond chains rigidify such a monolayer. The emission of porphyrin 1 remained after the self-assembly process. 1 was now localized on the bottom of a rigid membrane gap. Its fluorescence was again quantitatively quenched by the tetracationic manganese(III) porphyrinate 2, which fit in with the membrane gap. A larger manganese(III) porphyrin with a phenyl spacer between the porphyrin and methyl pyridinium rings could not enter, and no quenching was observed. The same experiment with a more fluid membrane made of octadecanethiol showed no such discriminating effect. The entrapment of 1,2 trans-cyclohexanediol within the "immobile" water volume of the membrane gap is also reported. Water-soluble compounds have thus been separated within a 2 nm(3) water volume from bulk water. So far, the membrane pores with a porphyrin bottom resemble natural enzyme clefts.