This paper describes shock‐tube studies of the optical index of refraction of ionized argon in which observed optical effects have been related to the ionization of the shocked argon. The plasma dispersion formula has been applied to the measured refractivities to determine values of equilibrium electron concentrations on the order of 1017cm−3. Good agreement is found between experimental and calculated values of the degree of ionization. The ionization relaxation process is strikingly portrayed in the interferograms and may be studied in some detail. Some questions concerning proposed kinetics for the relaxation behind strong shock waves in monatomic gases are discussed.