In a previous work, sodium alginate dense membranes demonstrated a good combination of selectivity and permeation flux for dehydrating organic solvents by pervaporation. In this article, the pervaporation performances of alginate composite membranes has been investigated to find out the best condition of membrane formation and the optimum operating conditions. Some ultrafiltration membranes made of poly(vinylidene fluoride), polyacrylonitrile, and hydrolyzed polyacrylonitrile-either commercially available or prepared in our laboratories--were used as supports for the composite membranes. Sodium alginate dense membranes, modified through ion ex change of sodium with multivalent metal ions (such as Al3+, Cr3+, Fe3+, and Mg2+) have also been prepared, and their permselectivities have been tested for the water-ethanol mixture. An interesting stability of the modified membranes in long-term operation is expected.