X-ray spectroscopy and imaging over a broad spectral range (0.1–20 nm) require transmission diffraction gratings and zone plates of high Z (atomic number) materials with narrow linewidths and large height-to-width ratios, often ≥10 to 1. Such structures have been achieved in gold and silver by electroplating into PMMA molds produced by x-ray lithography. Calculations of the x-ray attenuation and phase shift as a function of wavelength and Z show the value of expanding beyond Au and Ag the range of materials that can be used in diffraction gratings and zone plates. We present the result of calculations showing the effectiveness of W and Mo as substitutes for Au and Ag, respectively, and demonstrate reactive-ion etching of gratings in W and Mo with ∼80 nm wide slots, 0.4–0.7 μm deep. The etch gas CBrF3 was used which allowed us to achieve vertical sidewall structures, while comparative studies using CF4 and SF6 demonstrated excessive undercut. The W films were deposited by CVD and the Mo films by rf sputtering. This work is directed toward generating improved diffraction gratings for future x-ray astronomy missions, as well as improved x-ray masks for submicrometer lithography.