Hydrogen-deuterium (H-D) exchange is observed between hydrogen chloride and 1-ethyl-3-methylimidazolium ion in acidic mixtures of the molten salt,aluminum chloride:1-ethyl-3-methylimidazolium chloride. The exchange takes place at the 4.5-positions on the imidazolium ring, and it proceeds by an acid-catalyzed electrophilic substitution mechanism. The magnitude of the equilibrium isotope effect (K(EIE)) for this exchange process, which was found to be 1.5 +/- 0.3 in a 2:1 melt at 30-degrees-C, indicates that deuterium will accumulate in the imidazolium cation. The H-D reaction is first order in hydrogen chloride and seems to be first order in Al2Cl7-. The rate-limiting process for this exchange reaction appears to be protonation of the imidazolium cation by hydrogen chloride where Al2Cl7- acts as a chloride acceptor.