Abstract The proton-to-deuterium exchange reaction of the hydrogen atom at the 5-position of 15 pyrimidine derivatives has been studied. The exchange proceeds under both acidic and alkaline conditions. Under acidic conditions, the mechanism involves protonation at the 5-position (forming an σ complex), whereas under alkaline conditions the exchange is mainly a result of the formation of a tautomeric equilibrium, with one tautomer bearing an ...