An experimental study of the properties of supersonic plasma streams which have been seeded with small amounts of an electronegative gas is described. It has been found that striking reductions of the electron and ion densities of the plasma can be achieved using sulfur hexafluoride (SF6) as the seed gas. The properties of seeded argon streams at pressures of 1 Torr and velocities of about Mach 2 have been studied using double‐probe and microwave (9.2 Gc/sec) scattering techniques. The results show that when the SF6is introduced with a concentration of only 5 × 10−4relative to that of the argon in the stream, the plasma is quenched almost completely. Under these conditions the reflection of microwaves from the stream is reduced by 20 dB and the probe measurements show the presence of the ``hole'' cut in the plasma by the SF6. Measurements of seeding with argon, nitrogen, and oxygen in the argon streams are also discussed.