X-ray induced fluorescence is demonstrated as a novel and fast method for measuring vapor pressures at high temperatures and high pressures. As such, it is an excellent complement to the effusion method, which is limited to lower pressures. High-energy synchrotron radiation was used to measure the total densities of Dy in the equilibrium vapor over condensed DyI3 and Tm in the equilibrium vapor over condensed TmI3. Corresponding vapor pressures were determined with measured vapor cell temperatures across a range of vapor pressures of nearly three orders of magnitude, from less than 10(2) Pa to more than 104 Pa. Individual data points were obtained in time periods ranging from 10 to 30 s each. Published by Elsevier B.V.