X-Ray reflectivity has been used to investigate the microstructural changes of solution-cast stearic acid films before and after immersion in CoCl, solutions. Before immersion, the films possess a well-defined layered structure with an interlayer spacing of 4.01 +/- 0.05 nm. After the films were immersed in the CoCl, solutions, a new set of equidistant diffraction peaks emerge, the corresponding interlayer spacing of which is 5.13 +/- 0.05 nm. The X-ray photoelectron spectra of the films indicate the existence of cobalt ions inside the films after immersion. It is concluded that the permeation of the cobalt ions into the hydrophilic interlayer causes the stearic acid molecules to reorient perpendicular to the films, resulting in the increase of interlayer spacing and the roughening of the interfaces. (C) 2003 Elsevier B.V. All rights reserved.