Keggin-type polyoxometalates are effective catalytic materials for acidcatalyzed reactions and oxidation reactions. In this paper, we describe the fabrication of a porous membrane of Keggin-type polyoxometalates at the gas-liquid interface and the structural characterization of the membrane via aberration-corrected scanning transmission electron microscopy. The membrane is formed by the introduction of n-octylamine vapor into aqueous solution of Keggin-type silicotungstic acid (H4SiW12O40) and cesium chloride. The membrane obtained has the cubic structure of cesium salts of silicotungstic acid (CsxH4-xSiW12O40), exposing the (110) crystal plane preferentially. The single-unit cell layer of the upper surface of the membrane is densely packed with Keggin anions, while the lower layers are rich in vacancies at both the anion and cation sites. This structure results from the fast formation of the single-unit cell layer at the surface of the solution and slow stacking of Keggin anions under the layer.