High Quality Content by WIKIPEDIA articles! The kinetic isotope effect (KIE) is a dependence of the rate of a chemical reaction on the isotopic identity of an atom in a reactant. It is also called "isotope fractionation," although this term is somewhat broader in meaning.An isotopic substitution will greatly modify the reaction rate when the isotopic replacement is in a chemical bond that is broken or formed in the rate limiting step. In such a case, the change is termed a primary isotope effect. When the substitution is not involved in the bond that is breaking or forming, a smaller rate change, termed a secondary isotope effect is observed. Thus, the magnitude of the kinetic isotope effect can be used to elucidate the reaction mechanism. If other steps are partially rate-determining, the effect of isotopic substitution will be masked.