Convenient, tunable, sealed‐off cavity resonators are described at frequencies of 1, 2, 9, and 24 kMc, for use with internal pressures up to 1000 psi. The modes of oscillation employed were TE01n, TE11n, and TM01hybrid, all in a circular cylinder. A convenient method for sealing an iris, applicable over a wide‐frequency range, is to insert a tapered Teflon plug. A Kovar‐glass seal for coaxial lines is described that provides a good impedance match in a 50‐ohm line. The measuring procedure is described. Accuracy of dielectric loss measurements was increased by calibrating the loss changes of the cavity arising from reresonating it upon admission of lossless gas.