The objective is to find new materials for the fabrication of X-ray mirrors with high reflectance at 45 Angstrom wavelength at normal incidence. We have examined multilayers of inconel and carbon, which have high theoretical reflectivities. Planar magnetron sputtering was used to fabricate mirror samples (period = 22.5 Angstrom) on silicon substrates. Our best 30-period inconel/carbon multilayers had a reflectivity of 0.3%, which was about a factor of 20 below the theoretical maximum (5.5%). Experimental details presented here include our use of an in situ ellipsometer to monitor the thickness of the layers and to provide information on the early stages of film growth and interdiffusion at the interfaces. Interdiffusion was found to occur at the carbon-on-inconel interface because of the rough underlying inconel layer, but not at the inconel-on-carbon interface because of the smooth underlying carbon layer. Interdiffusion at the carbon-on-inconel layer was also found to increase with the number of layers. The accuracy of ellipsometry for monitoring film thickness during deposition was limited by the theoretical model used for the optical constants of the film. As a result, we were not able to improve on the standard technique of ’timing’ for control of layer thicknesses.