Résumé : From measurements on hyperpure and commercially pure low- to high-loss liquids over a wide spectral range and using a variety of experimental equipment, we have been able for the first time to quantify both the systematic and the random uncertainties to which extra-high-frequency dielectric measurements are subject. From our measurements it has become possible to specify certain liquids and solids as standard reference materials for the calibration of high-frequency dielectric measuring equipment. Copyright © 1980 by The Institute of Electrical and Electronics Engineers, Inc.