The dissociation of oxygen has been observed behind shock waves at Mach numbers of 8 to 10, over a temperature range of 3000–5000°K. Photographs of the shocks were made using a Mach‐Zehnder interferometer and a spark light source of effective duration 0.1 &mgr;sec. The velocity of the shock was measured using thin‐film resistance thermometers to detect its passage. From this velocity and the state of the gas ahead of the shock, one can calculate the state of the gas behind the shock, both in the reaction zone and at equilibrium, from the interferometrically observed density ratios. Up to 12% equilibrium dissociation was obtained, following reaction zones of the order of 1 cm in length. The following rate constant, obtained from classical collision theory for the principal reactionO2+O2&rlhar2;kD2O+O2,fitted the data well:kD= 7.4 × 1011PT½(59 380/T)3exp (−59 380/T) cc mole−1sec−1, where 59 380 is the known dissociation energy of oxygen, expressed in degrees Kelvin. The quantityP, the ``collision efficiency,'' was adjusted to fit the data, and was found to be 0.07. The data are consistent with the results of several other investigators who used different methods. The recombination rate constant calculated from the dissociation rate constant is 8.4 × 1014cc2mole−2sec−1at 3500°, and is approximately proportional toT−2.