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The value of the long lifetime for positron annihilation in a number of organic liquids is markedly reduced by the presence of dissolved oxygen or nitric oxide. If the change in long lifetime is interpreted as a quenching reaction of triplet positronium formed in these liquids, the reaction with oxygen or nitric oxide is found to obey bimolecular kinetics with quenching rate constants in the range 1010−1011M−1sec−1, about twice those for oxygen quenching of molecular excited states in these same liquids. These reactions also exhibit the usual features of diffusion controlled reactions having inverse viscosity dependence and low activation energy. The interpretation of these and other properties of positronium in liquids is made by postulating solvent-cage trapping of the triplet positronium. These considerations severely limit the amount of information which may be obtained by study of these reactions in liquids.