Through a simple hydrothermal and thermal decomposition process of the ammonium aluminum carbonate hydroxide (denoted as AACH) precursors, we prepared mesoporous gamma-Al2O3 fibers with tunable diameters by manipulating the amount of urea (pH-adjuster) in the reaction mixture (i.e., pH control). The experimental results show that the diameter of the obtained AACH fibers decreased obviously when the pH values of the reaction mixtures changed from 2.5 to 5.0. The as-formed precursors could transform to the morphology-remained cubic gamma-Al2O3 with a slight shrinkage in diameter of the fibers after an annealing process. N-2 adsorption-desorption experiment indicates that the as-synthesized alumina microfibers and nanofibers have large surface area (176 and 230 m(2)/g, respectively) and narrow pore-size distributions. Both the gamma-Al2O3 microfibers and nanofibers are powerful in the removal of Congo red pollutant from waste water. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.