The vibrational frequency of the I-2(-) chromophore in size-selected I-2(-)(Ar)(n) and I-2(-)(CO2)(n) clusters has been measured to wave-number accuracy. The frequencies are determined by creating a coherent superposition of vibrational levels with a femtosecond laser pulse via a resonance impulsive stimulated Raman scattering process. The resulting wave-packet oscillations are detected with femtosecond photoelectron spectroscopy. Blueshifting of the frequency occurs upon solvation, with larger shifts observed for solvation with CO2. The nature of the shifting is discussed and related to specific cluster geometries.