Acicular hydrogoethite (alpha-FeOOH.xH(2)O; 0.1 < x < 0.22) particles of high aspect ratio of greater than or equal to10 and narrow size distribution with length similar to0.5 mum are prepared by air oxidation of Fe(OH)(2).xH(2)O (80 < x < 120) in presence of morphology controlling cationic additives (0.01 at.% of Pd2+ or Rh3+). Growth of hydrogoethite particle proceeds by the oxidation of Fe(OH)(2).xH(2)O to an intermediate phase through the nucleation within the amorphous ferric oxy-hydroxide [FeOx(OH)(3-2x).yH(2)O] and growth of alpha-FeOOH.(H2O)(x) acicular particles with the aid of morphology controlling agents, as evidenced from X-ray diffraction (XRD) and transmission electron microscope (TEM) studies. Preferential adsorption of additives on certain crystallographic planes and thereby retarding the growth in the perpendicular direction, allows the particles to acquire acicular shape with high aspect ratio similar to10. Thermal analyses of hydrogoethite samples show excess weight loss (similar to3-4%) compared to goethite samples. Presence of H-O-H vibrational absorptions arising from strongly coordinated water molecules in hydrogoethite and its absence in goethite is evidenced from IR spectra. H2O molecules in hydrogoethite are located in the strands of channel formed in between the double ribbons of FeO6 octahedra running parallel to c-axis of goethite, wherein the water molecules are bridging between the nearest neighbour cations. (C) 2002 Elsevier Science B.V. All rights reserved.