epsilon-Al-13 (AlO4Al12(OH)(24)(H2O)(12)(7+)) is a tridecameric polymer with a 1-2 nm-sized aluminum polyoxocation. The sol with a high concentration of 80% epsilon-Al-13 (2.0 M total Al concentration) is prepared by use of chemical synthesis and membrane distillation (MD) concentration process. The epsilon-Al-13 or Al-b content in the final sol is highly influenced by the pH of the synthesized sol. As the MD proceeds, the pH of the synthesized sol increases and Al-c or Al-30 decomposes into oligomers which belong to Al-b but not epsilon-Al-13. Therefore, the content of Alb increases but the content of epsilon-Al-13 remains constant. (C) 2009 Elsevier B.V. All rights reserved.