The intermolecular triplet-triplet (T–T) electron energy transfer discovered by Ermolayev and Terenin in solid solutions and adsorbates1,2was later observed in various aggregate states: in liquid solutions of organic compounds3, in crystals4and vapours5,6. It has become rather a precise and effective method which allows one to study the properties of excited states of molecules and processes proceeding with their participation7. The use of the T–T-transfer for studying paths of degradation of excitation electron energy made it possible to considerably advance the problem of radiationless transitions in molecules8. Although the process of the T–T-energy transfer in all the media is conditioned by the exchange-resonance interactions9, its kinetics is different depending on the aggregate state. This paper deals with the regularities of the T–T-energy transfer in gas phase systems.