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The frequency shift of a lattice resonant mode associated with the isotopic substitution of an impurity ion has been measured for NaCl: Cu+, KI: Ag+, and KBr: Li+. For all three systems, the experimental frequency shifts can be described satisfactorily by an Einstein oscillator representation for the resonant mode with the oscillator mass set equal to the impurity-ion mass. The model of Benedek and Nardelli and also Klein does not account for the experimental results of NaCl: Cu+ or KBr: Li+. The inclusion of anharmonic forces does improve the agreement between theory and experiment, but a large discrepancy still exists for KBr: Li+.