Thermal treatment of a Mo/Si multilayer stack enhances its reflectivity in the soft x-ray region. The multilayer x-ray mirrors are fabricated by electron beam evaporation in ultrahigh vacuum. In situ measurement of the reflectivity during the deposition allows thickness control and an observation of changes in quality of the boundaries. By heating the substrates during deposition we obtain a smoothing of the interfaces. This leads to x-ray mirrors with peak reflectivity around 50% for normal incident radiation of wavelengths between 130 and 140 A.