The velocity of sound was measured in dissociating dinitrogen tetroxide. These measurements were made over a frequency range of 20 to 800 kc and pressure range of 1 to 70 cm Hg giving a frequency/pressure range of 0.0217 to 60.8 Mc/atm. The temperature range covered was from 0° to 50°C. At 20°C and 10 cm Hg the velocity change was 4% over the frequency range stated and the relaxation time was 1.72 μsec.