A Kerr‐cell shuttered spectrograph has been used to make measurements of the state of the plasma produced by an electric shock tube operating at high pressure (100 Torr of CO2). Relative line intensities have been used to measure the temperature and Stark broading has been used to obtain the electron densities. The free‐running plasma front is expected and found to be in local thermodynamic equilibrium. The complete state of the plasma flow is then deduced. Measured pressure, trajectory and density profiles approximate blast‐wave calculations with, however, higher‐pressure and lower‐density values at the front than calculated from the Rankine‐Hugoniot theory. The temperature profile does have a rising trend upstream, but the front temperature is much too high (a factor of four). Violent forward mixing of the arc gas, due to the deceleration of the front, accounts for this. An arc‐produced photon precursor is assumed to dissociate the CO2.