The optical refractive index has been measured interferometrically behind Mach 6 to 9 shock waves advancing into nearly equimolal hydrogen and oxygen at pressures of 0.01, 0.015, and 0.02 atm. Pressure measurements in these experiments together with other data on similar systems show the shocked gas to be in an equilibrium state. The refractivity to be expected from all species present, except the hydroxyl radical, is computed, and the difference between this and the observed refractivity is attributed to hydroxyl radical present at the equilibrium density. The resultant specific refractivity in the blue is K = 0.35 ± 0.04 cm3/g, corresponding to a polarizability of ᾱ = 1.5 ± 0.2A3 and a refractivity of μ − 1 = (2.5 ± 0.3) × 10−4.