Measured values of electron density and collision frequency in shock‐heated argon are presented and compared with theoretical equilibrium values. Both parameters are measured as a function of time during a single shock tube experiment. The measured density at 1 and 3 Torr forepressure equals or exceeds the predicted 1016−1017electrons per cc at high Mach number (15–18); for lower Mach number, measured values fall far below the calculated curve, showing that loss mechanisms dominate thermal electron production over a significant interval of shock wave flow. The measured collision frequency compares well with the predicted value at 1 Torr but is lower by about an order of magnitude at 3 Torr. The experimental configuration comprises a theta pinch driven shock tube designed to drive a shock front into a coaxial waveguide. Two types of interference patterns are observed by transverse sampling of the longitudinal waves: the first, an anomalous pattern, is interpreted as a resonance caused by electrons ahead of the shock; the second, a well‐defined interference between incident and reflected waves, yields measurements of the plasma reflection coefficient magnitude and phase.