The use of kinetic isotope effects (KIEs) for the short-lived radionuclides ¹¹C and ¹⁸F in the study of reaction mechanisms is described using some examples. Leaving group fluorine KIEs (k ¹⁸/k¹⁹) have been utilized to determine the rate-limiting step for nucleophilic aromatic substitution reactions (S_NAr). The fluorine KIE was also used to probe the effect of changing solvent and nucleophile steric hindrance on rate-limiting step. The mechanism for a base promoted elimination reaction was determined to be stepwise (E1cB) by a multiple KIE study including the leaving group fluorine KIE. The transition state structures for aliphatic nucleophilic substitution reactions (S_N2) have been investigated by multiple KIE studies for cases where the substrate substitution, leaving group or solvent has been varied. Carbon KIEs for labelled α-carbon atom in the substrate are large, k¹¹/k¹⁴ = 1.189-1.220. For labelled nucleophile cyanide ion. k¹¹/k ¹⁴ = 0.99951-1.0119.