The effect of aging on Co/C multilayer x-ray mirrors is presented. One significant result is the enhancement of the reflectivity at grazing incidence with time for the Co/C multilayers with C-on-top stored in air or argon and Co-on-top stored in argon. This can be interpreted in terms of a Co–C phase-separation at Co–C interfaces due to the positive enthalpy of Co–C mixing. Results also show that oxidation of the surface of Co-on-top Co/C multilayers plays an important role in the decrease of reflectivity. The oxidation can be prevented by storing the multilayers in an oxygen-free atmosphere or by depositing amorphous carbon as the top layer. The reflectivity of tarnished multilayers can be restored by removing the oxides by wet chemical methods. ©1997 American Institute of Physics.