A quartz and metal seal capable of holding compressed gases at 1500 atmospheres forms the tapered, coaxial transition leading from a standing‐wave detector or from a bridge, into a coaxial condenser. The continuity of characteristic impedance through the seal is sufficiently good to allow accurate measurement of dielectric constant and loss in compressed gases and liquids from low frequencies to 3000 megacycles (&lgr;0=10 cm) in a single cell. The performance is illustrated with isotherms of the dielectric constant of N2O through the critical point. Organic sealing materials are not used. The unit promises to be satisfactory for a wider temperature range than is reported here.