Three different experimental techniques are applied to the measurement of physical parameters involved in the determination of the optical performances of multilayers designed for soft X-ray mirrors. Geometrical parameterssuch as single layer thicknesses, multilayer period and interface roughness are evaluated by using soft X-ray reflectometry during the deposition process and grazing incidence X-ray reflectometry with the Cu K, radiation after the multilayer fabrication. The growth mechanism and surface composition of ultra-thin films constituting the multilayers are obtained by Auger electron spectroscopy performed during the layer deposition. The whole of these measurements is applied to elaborate a modelisa-tion of the real multilayers in view of their optical applications.