The thermal decomposition of alpha-FeOOH, prepared by air oxidation of Fe(OH)(2) at room temperature, was investigated by means of Mossbauer spectroscopy and transmission electron microscopy. The measurements were performed on powder samples previously annealed between 200 and 700 degrees C. From Mossbauer spectral data the reaction isotherms, rate constants and the activation energy of the process were determined. The transformation into alpha-iron oxide takes place preserving the needle-like shape of particles. An Avrami-Erofeev mechanism was identified to characterise the solid phase transformation of alpha oxyhydroxide to alpha iron oxide.