An improved wavelet transform method for the analysis of specular X-ray reflectivity data has been developed. It permits the evaluation of the thickness and roughness of particular layers in the thin film without assuming a certain film structure. The advantage of this method is that it can be applied to the analysis of a multilayer with unknown chemical and physical properties. It is useful in the characterization of structures with a complex composition, particularly oxide or diffuse layers. The present approach was successfully applied to the study of experimental data obtained on Gd thin nanoparticle films. (c) 2004 Published by Elsevier B.V.