The central portion of the plasma produced in a shock tube was sampled by allowing it to flow into a length of X‐band rectangular waveguide, coaxial with the shock tube. The temperature of the portion of ionized gas inside the waveguide was determined from the measurement of the microwave noise emitted by the shock‐heated gas and its absorption coefficient measured at the same frequency, 9.05 Gc/sec. The measured temperatures were between 4700 and 6000°K for shocks of Mach numbers between 7.0 and 10.6 in argon at an initial pressure of 4 mm Hg. The selection of the middle axial portion of the shock eliminates the boundary layer present at the walls of the shock tube.