Shock‐tube and discharge techniques make it possible to generate in the laboratory significant volumes of dissociated and/or ionized gases. This paper describes some studies of the optical index of refraction &mgr; of dissociated diatomic gases. The physical interpretation of specific refractivityK= (&mgr; − 1)/&rgr; is discussed and related to polarizability, and various methods of computing such quantities are reviewed briefly. Shock‐tube experiments are described in which optical interferometry has been utilized to measure the polarizability of atomic oxygen and nitrogen with the result&agr;¯(O I)=0.77±0.06 A3and&agr;¯(N I)=1.13±0.06 A3. The use of optical interferometry for determination of gas density has been justified for experiments involving air at temperatures to at least 5000°K.