The Swain equation relates the kinetic isotope effect for the protium/tritium combination with that of the protium/deuterium combination according to:

${\displaystyle {\frac {k_{H)){k_{T))}=({\frac {k_{H)){k_{D))})^{1.442))$

where k_H,D,T are the reaction rate constants for the protonated, deuterated and tritiated reactants respectively.