The explosive oxidation of acetylene, initiated homogeneously by the flash photolysis of a small quantity of nitrogen dioxide, has been investigated by flash spectroscopy. The absorption spectra of OH, CH, C
2
(singlet and triplet), C
3
, CN and NH, a number of which have not previously been observed, are described, and the relative concentrations, at all times throughout the explosion, are given. Four stages have been distinguished in the explosive reaction: 1. An initial period during which only OH appears. 2. A rapid chain branching involving all the diatomic radicals. 3. Further reaction, occurring only when oxygen is present in excess of equimolecular proportions, during which the OH concentration rises exponentially and the other radicals are totally consumed. 4. A relatively slow exponential decay of the excess radical concentration remaining after completion of stages 2 and 3. The duration of stage 1 is 0 to 3 ms. In an equimolecular mixture at 20 mm total pressure, containing 1.5 mm NO
2
, the durations of both stage 2 and stage 3 are approximately 10
-4
s and the half-life of OH in stage 4 is 0.28 ms. A preliminary interpretation of these changes and of the radical reactions is given.
Plasma Kinetic Theory