Rate constants for the quenching of O2(1Σg+) in the gas phase have been determined for 12 aliphatic hydrocarbons. The values are discussed in terms of a mechanism in which the magnitudes of the rate constants are determined by the abilities of the quenchers to become vibrationally excited when O2(1Σg+)is relaxed to O2(1Δg).The resulting equations are tested quantitatively by relating the rate constants to the absorption spectra of the quenchers at the frequencies of the O2(1Σg+−1Δg) transition.