The cross-interaction constant, ρij, is a second derivative parameter representing a change in the intensity of interaction between the two reacting partners (or fragments) i and j in a reaction. It can be either an activation or a reaction parameter. The sign and magnitude of ρij have important mechanistic significances and are useful as a tool for the studies of organic reaction mechanism. We have presented several more important applications of the cross-interaction constants. A review with 42 references.