Multilayer X-ray mirrors have been deposited by triode dc sputtering using a new method of thickness monitoring which is based on the dependence of the deposition rate on the target current. Thicknesses can be controlled with an accuracy of better than 0.1 〈. High efficiency W-C, and Ni-C multilayer mirrors have been synthesized and tested at 1.54 Å (Cu Kα) and 44.79 Å (CK,α). Absolute reflectivity measurements at λ = 44.79〈 (CKD) have been carried out. In this case the incident beam is previously polarized by a premonochromator equipped with a pair of parallel plane multilayer mirrors fixed at an angle close to Brewster's one (0---,45°). Thus the measured reflectivities are not affected by a progressive variation of the P component.