The heterodyne beat frequency method of determining dielectric constants has been applied to the measurement of the dielectric constant of air with an applied frequency of field of 900,000 cycles per second. Radio broadcast signals, stripped of modulation, were used as sources of alternating current of constant high frequency. An electron coupled oscillator was used in the circuit containing the test condenser. Both sources of high frequency were isolated from the detector through buffer stages of amplification. Determination of the beat frequencies was made by photographic recording with a two string oscillograph. A dielectric constant of 1.00058986±0.00000050 is found for air at normal pressure and temperature. This result is in close agreement with the results of other investigators for lower frequencies of applied field and for static measurements.