The effect on kinetics of the substitution of one isotope for another in a bond that must be broken in a rate-limiting step (primary isotope effect); especially prominent in the case of cleavage of a bond to a deuterium atom compared with a hydrogen atom. Smaller effects are seen in the case of a substitution of an isotope for a non-reacting atom (secondary isotope effect); effects are also seen on parameters other than the rate-limiting step.