Experimental results of applying the method of two‐wavelength, streak interferometry are given for the cylindrical, time‐dependent flow of ionized argon produced by an exploding wire. Measurements from simultaneous streak interferograms at wavelengths 5460 and 4358 Å are used to determine contour charts of electron concentration and mass density of the shock‐arc‐heated argon plasma. Where thermal equilibrium can be assumed, values of velocity, pressure, and temperature can be deduced. The resulting temperatures agree well with those obtained from the Saha relation at the measured degree of ionization. The reduction equations and assumptions needed for the axisymmetric analysis are given. An estimate is made of the error in measured mass density caused by multiple ionization.