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 @E02142@ and @E02150@ sometimes satisfy the relation: \[\Delta ^{\ddagger}H - \beta \ \Delta ^{\ddagger }S = \text{constant}\] where the parameter \(\beta \) is independent of temperature. This equation (or some equivalent form) is said to represent an 'isokinetic relationship'. The temperature \(T=\beta \) (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 \(\Delta ^{\ddagger}H\) with \(\Delta ^{\ddagger}S\) are often spurious and the calculated value of \(\beta \) is meaningless, because errors in \(\Delta ^{\ddagger}H\) lead to compensating errors in \(\Delta ^{\ddagger}S\). Satisfactory methods of establishing such relationships have been devised.