The last two decades have been marked by a considerable activitity in the field of gas-phase ion chemistry. As a result a large amount of data on the thermodynamics of ion-molecule interactions has appeared in the literature. The interpretation of these data often required the assistance of ab initio molecular orbital theory, in this review we present a compilation of the most significant contributions of our research group to the study of the chemistry of gas-phase ions. There is obviously much excellent work of other groups that will not be presented here.