The effects of urotropin (hexamethylenetetramine) on the chemical and microstructural properties of Fe-oxide powders, prepared by FeCl 3 hydrolysis in aqueous solutions at 90°C, were investigated using X-ray powder diffraction, transmission electron microscopy, Fouriertransform infrared spectroscopy (FTIR) and 5 7 Fe Mossbauer spectroscopy. In the absence of urotropin, akaganeite (β-FeOOH) particles showed different morphologies, such as spindles, stars, X- and Y-shapes. After 5 h of aging and for an initial 0.025 M urotropin concentration, acicular β-FeOOH particles (monodispersed) of decreased size were obtained. For the same aging time and 0.25 M urotropin the particles were amorphous for X-ray diffraction, while 5 7 Fe Mossbauer spectroscopy indicated that these particles were of very small dimensions (super paramagnetic) and probably of the FeOOH structure. The β-FeOOH formation at pH values significantly higher than pH = 2 was explained by incomplete substitution of Cl - ions in Fe(III)-hydroxy polymers with OH - ions. No evidence of specific adsorption of organic components on the solid hydrolytical products was detected by FTIR spectroscopy. After 7 days of aging and for an initial urotropin concentration of up to 0.025 M, β-FeOOH was also obtained as a single phase, while for c ≥ 0.1 M urotropin mixtures of goethite (α-FeOOH) and haematite (α-Fe 2 O 3 ) were formed. Phase transformations β-FeOOH → α-Fe 2 O 3 and β-FeOOH → α-FeOOH → α-Fe 2 O 3 were discussed in the sense of the dissolution/precipitation mechanism.