Compressibility and expansibility of activation, ${Delta}K^{ eq};and;{Delta}E^{ eq},$ are defined with the use of general principles that an activation parameter is the difference in partial molar quantities of the parameter for the transition and initial state. Two related parameters, (${partial}{Delta}H^{ eq}/{partial}P)_{gamma};and;{Delta}W^{ eq}(=P{Delta}V^{ eq}),$ are also derived. Simpler interpretation of the existing kinetic data are possible with these activation parameters, while other derivations lead to complicated expressions of no practical significance.