The rejection characteristics of three types of alumina membranes for polyethylene glycol (PEG) polymers and dextrans soluted in water have been investigated. If the temperature at which the membranes are treated is increased, the pore size increases and as a result the cut-off value also increases. The Al2O3-400 membrane (i.e., the membrane treated at 400°C) exhibits a cut-off value of 2,000 for PEG while that of an Al2O3-800 membrane is about 20,000. Both Al2O3-500 and Al2O3-800 membranes show rejections higher than 80% for the dextrans investigated. The rejection behaviour is compared with literature data on Nuclepore membranes. The performance of the alumina membranes is compared with that of other inorganic membranes. Low molecular weight solutes show only small rejection values solar, although high values are reported in the literature for Vycor glass membranes with pores of similar size.