The annealing e ects on thin Cr layers deposited by rf sputtering are investigated by X-ray re¯ectivity and glancing-incidence di raction. It is shown that annealing induces a phase segregation which dramatically changes the features of the X-ray re¯ectivity pattern. This study allows us to present and discuss the main features of X-ray re¯ectivity, which make it a powerful tool for the non-destructive analysis of thin single layers or multilayers and interfaces.