Abstract : Atomic collisions in solids induced by ion beam are often associated with the concept of disorder. In fact, the mobility induced in solids by ion irradiation at appropriate temperatures leads to the production of a wealth of phases which may (or not) be related to the equilibrium phase diagram. Despite, many attempts are made to understand the phase stability and the enhanced mobility of defect under irradiation even at the atomic scale, no clear picture of ion beam mixing exists. The major problem associated with ion beam mixing comes from the fact that it remains quite difficult to accurately measure a concentration over few nanometers. The X Ray Reflectometry (XRR), extensively used in micro electronics, appears to be a useful technique to overcome this difficulty. In this work, we apply the XRR technique to study the nanostructuration of Cr/Si layers induced by 80 keV Kr ion beam at room temperature, a textbook example of ion beam mixing. The analysis of XRR profiles allows computing accurate profiles of Si and Cr concentrations with a resolution equals to 1 nanometer. From these experimental profiles, we point out that the ion beam mixing appears to be a complex process which can not be only described as a diffusion controlled process.