An ultrahigh vacuum deposition-reflectometer system was constructed to allow the deposition and subsequent irt situ reflectance measurement of extreme-ultraviolet multilayer mirrors. In this article various aspects of the deposition chamber and the reflectance measurement setup are discussed. To demonstrate the capabilities of this system, Y/Mo multilayer mirrors were studied in detail by varying deposition conditions and measuring their subsequent in situ reflectance. By optimizing the deposition conditions, reflectances of 21.3%, 34.7%, and 46.1% were obtained for the Y/Mo mirrors at wavelengths of 7.9, 9.7, and 11.4 nm, respectively.