at high temperatures would proceed significantly before vibrational equilibration would occur. The purpose of this investigation was to determine how the dissociation rate will be affected by a lack of vibrational equilibrium.Studies of the dissociation rate of dilute 02 -Ar mixtures were made in a 24" diameter shock tube from 5000 -18000 0 K. The 02 concentration was monitored by its absorption of 1470A radiation. AnArrhenius plot of the data yielded a straight line from 5000 -U000 0 K, the rate constant being given by kd = 2.9 (+ 12%) x 1014 exp (.D/RT) cc/mole-sec. Above 110000 the data deviate from the line given by this equation -at 18000° kd being . 45 times the calculated value.An incubation time, At, was observed during which dissociation does not proceed to a significant extent. The ratio of this incubation time to the vibrational relaxation time (obtained by extrapolating Camac's low temperature results) when plotted against translational temperature, displays a slight negative temperature dependence. At 18000 A t/T v = 0. 4, at 80000 At/Tv =1, and at 5500 0 we estimate that At/Tv 2.-ii-