All-inorganic membranes that are composed entirely of sepiolite, a naturally occurring Mg-silicate clay mineral, have been fabricated. Pinhole-free, self-bonded membranes with transecting pores in the large mesopore to macropore size range were obtained. Characterization of the thermally treated membranes by PXRD has revealed that the sepiolite is transformed into sepiolite anhydride' by heating in air at elevated temperatures. SEM studies have subsequently shown that the air dried membranes do not develop cracks or other similar flaws even after being subjected to temperatures as high as 750 degreesC for several hours. Calcined sepiolite membranes were tested for the ultrafiltration of high molecular weight poly(ethylene glycol) and poly(ethylene oxide) molecules in aqueous solution. The MWCO was determined to be close to 300 000 for all of the membranes that were studied in the preliminary tests. Significant membrane fouling was observed, which dramatically reduced the permeation flux rate during the ultrafiltration tests.