Abstract: The energetics, rate and mechanism of gas phase ion-molecule associationreactions have been areas of substantial activity since the early 1960'S. Asso-ciation reactions are important because of the role clustering plays in the che-mistry of the atmosphere, ionosphere and interstellar clouds as well as ingas to liquid phase transitinos. This paper is organized as follows: (1)A bri-ef outline of ion-molecule association reactions; (2)Thermodynamics: Chemicalequilibrium and △H°, △S°; (3)Chemical kinetics and statistical phase spacetheory. Meanwhile some experimental examples will be described.