We have measured the permittivity of representative samples of natural seawater, synthetic seawater, and aqueous NaCl solutions over the frequency range 3–20 GHz, in 0.1-GHz steps and over the temperature range −2°–30°C in 1° steps. Additional measurements have been made at spot frequencies (23.8, 36.5, and 89 GHz) and at selected temperatures between −2° and 3O°C. The data from these measurements have allowed us to deduce an interpolation function for ε(υ t, S) in the ranges 2 ≤ υ ≤ 20 GHz, −2° ≤ t ≤ 30°C, and 20‰ ≤ S ≤ 40‰ with a precision of 1%. If the frequency range is extended up to 40 GHz, the precision of the interpolation function is about 3%.The data have also allowed us to compare the permittivities of natural seawater, synthetic seawater, and aqueous NaCl solution with the same salinities. Natural and synthetic seawater have the same permittivities within a 1% experimental error estimate. An aqueous NaCl solution has a significantly different permittivity (up to about 6% difference, depending upon the frequency and temperature).