The development of surface roughness of two synthetic goethites with a high, 176 m(2)/g, and a low, 7 m(2)/g, initial specific surface area by acid attack is described in terms of fractal geometry. The dissolution kinetics were influenced by the presence of micropores and small mesopores created during dissolution and leading to an initial increase in surface area causing S-shaped dissolution curves. The fractal dimensions were calculated from the adsorption isotherms obtained with N-2 as adsorbate. These values of surface fractal dimension agree quite well with the values of the mass and surface fractal dimension obtained with small angle X-ray scattering (SAXS).