When a series of structurally related substrates undergo the same general reaction or when the reaction conditions for a single substrate are changed in a systematic way, the enthalpies and entropies of activation sometimes satisfy the relation

where the parameter β is independent of temperature. This equation (or some equivalent form) is said to represent an "isokinetic relationship". The temperature T = β (at which all members of a series obeying the isokinetic relationship react at the same rate) is termed the "isokinetic temperature".

Supposed isokinetic relationships as established by direct correlation of Δ^†H with Δ^†S are often spurious and the calculated value of β is meaningless, because errors in Δ^†H lead to compensating errors in Δ^†S. Satisfactory methods of establishing such relationships have been devised.