We report a significant increase of the reflectivity of a soft X-ray Co/C multilayer at grazing incidence after low temperature annealing. By monitoring the enhancement of the first order modulation peak on annealing, the effective interdiffusion coefficient as low as -10－25ｍ2ｓ－1 has been measured,which is nearly equal to the true macroscopic interdiffusion coefficient as the modulation wavelength of the Co/C multilayer studied is well above the critical wavelength. The negative true macroscopic interdiffusion coefficient indicates that there is a tendency to phase separation in the Co-C system, which can be interpreted as the positive enthalpy of mixing calculated based on Miedema′s macroscopic atom model. The decrease of reflectivity above some anneal temperature is also discussed and explained as the competition between interfacial sharpness and roughness.