The influence of plasma and thermal treatments on the structure and composition of sapphire (00.1) surfaces has been studied by hemispherically recorded x-ray photoelectron spectroscopy and diffraction (XPD) to better understand the surface conditioning during the growth initiation procedure for the epitaxy of GaN : A treatment involving an O-2 plasma generated by electron cyclotron resonance is shown to efficiently remove surface contamination and to produce well-ordered surfaces. After a thermal treatment in vacuum the sapphire (00.1) surface becomes terminated by Al atoms. ALN films with good short-range order are obtained by a simple high temperature nitridation step in the metal organic chemical vapor deposition reactor. A novel direct crystallographic method termed CHRISDA is employed for the analysis of the XPD data. By analysis of the XPD patterns the crystal structure of sapphire is found to extend into the probed near surface region without significant perturbations. The ALN layers formed by thermal nitridation were found to crystallize in the hexagonal phase and to be terminated by nitrogen atoms.