The influence of the aging in a highly alkaline medium at 160 degrees C on the microstructural properties of alpha-FeOOH was monitored by XRD, Fe-57 Mossbauer and FT-IR spectroscopies and FE-SEM. In the synthesis of acicular and monodisperse alpha-FeOOH particles, tetramethylammonium hydroxide was used as a precipitating agent. The crystal growth of alpha-FeOOH by the mechanism of lateral arraying was observed with prolonged aging time. Smaller alpha-FeOOH particles were dissolving with time, thus providing Fe(OH) ions for the additional growth of alpha-FeOOH particles and improvement of their crystal shape. The XRD analysis confirmed that the growth of alpha-FeOOH crystallites perpendicularly to the crystallographic c-axis was favoured. The Mossbauer spectrum of alpha-FeOOH showed a distribution of the hyperfine magnetic field (HMF) which gradually narrowed with a longer aging time. The average value of the HMF distribution was shifted from 35.3 to 37.1 T between 2 and 168 h of aging. Characteristic IR bands of alpha-FeOOH were sensitive to the microstructural changes of alpha-FeOOH particles during their aging in the suspension in a highly alkaline medium at 160 degrees C. (C) 2007 Elsevier B.V. All rights reserved.