The use of the short-lived radionuclides ¹¹C and ¹⁸F for the elucidation of organic reaction mechanisms is described. Examples of the different mechanistic problems that are discussed include concerted vs. stepwise base catalysed elimination (E2 or E1cB) and finding the rate limiting step for nucleophilic aromatic substitution (S_NAr). The use of radionuclides to learn details about transition state structure for concerted nucleophilic aliphatic substitution (S_N2) is also described.