The (1Δg)2 and 1Δg states of oxygen have been produced by direct laser (He–Ne and Nd–YAG) excitation of oxygen at high pressure dissolved in a suitable solvent, and their reactivities with various substrates investigated. For 9,10-dimethylanthracene and 1,3-diphenylisobenzofuran, the quantum yields for (1Δg)2 excitation are, within experimental error, twice those for 1Δg excitation. In the reactions of 1Δg oxygen with 9,10-dimethylanthracene and tetraphenylcyclopentadienone, both chemical reaction (rate kr) and physical quenching (rate kq) are observed. For 9,10-dimethyl-anthracene, kq/kr 0.6, and for tetraphenylcyclopentadienone kq/kr 1.7. With 1,3-diphenyl-isobenzofuran, kq/kr is in the range 0–1. It is shown unambiguously that 1Δg oxygen reacts with two tertiary amines, triethylamine and tropinone, in addition to being quenched. For triethylamine, kq/kr 11.