The performance of multilayer x-ray mirrors is improved by smoothing the boundaries within the multilayer stack with ion bombardment at grazing angles of incidence. The process is applied to Co-C and to RhRu-C mirrors for normal incidence telescopes at ? = 63.5 and 114 Å, and the reflectivity is increased by a factor of 2 in both cases. Accumulation of roughness during deposition is eliminated. An estimate of the presently achievable reflectivity for normal incidence mirrors in the ? = 44 to 120 Å wavelength range is given.