The reaction O(23P)+ CS2→SO + CS was initiated by flashing NO2+ CS2 mixtures with radiation restricted to wavelengths ≳3000 Å. Its rate was determined at 305 and 410°K, by monitoring the formation of CS by kinetic absorption spectroscopy. The rate constant k2 in 1. mole–1, sec–1, may be expressed by log10k2= 9.8 (±0.2)– 600(±300) cal (mole 2.303 RT).–1 Rate constants for the reactions of O(23P) with but-1-ene, isobutene and NO2 were determined relative to k2 by observing how the total amount of CS produced varied with the composition of the flashed mixture. CS was thereby employed as a spectroscopic “marker” whose yield depended on the relative importance of reactions competing for the atomic oxygen.