For most reactions the activation enthalpy and activation entropy are unknown, but, if these parameters have been measured and a linear relationship is found to exist (meaning an LFER was found to hold), the following equation describes the relationship between  and :
Inserting the Gibbs free-energy equation and combining like terms produces the following equation:
where  is constant regardless of substituents and  is different for each substituent.
In this form,  has the dimension of temperature and is referred to as the isokinetic (or isoequilibrium) temperature.
Alternately, the isokinetic (or isoequilibrium) temperature may be reached by observing that, if a linear relationship is found, then the difference between the s for any closely related reactants will be related to the difference between s for the same reactants:
Using the Gibbs free-energy equation,
In both forms, it is apparent that the difference in Gibbs free-energies of activations () will be zero when the temperature is at the isokinetic (or isoequilibrium) temperature and hence identical for all members of the reaction set at that temperature.
Beginning with the Arrhenius equation and assuming kinetic compensation (obeying ), the isokinetic temperature may also be given by
The reactions will have approximately the same value of their rate constant  at an isokinetic temperature.