Vibrational predissociation spectra of the (CH 3 ) 2 RSi + ·Ar n , (R = H and CH 3 , n = 1 and 2) ions are compared with harmonic calculations to structurally characterize these putative reactive intermediates. Although the vibrational photofragmentation behavior indicates that the Ar–Si bond is quite strong relative to that found in closed shell ions, formation of the Ar adducts is calculated to cause only minor perturbations to the intrinsic vibrational band patterns of the isolated ions. In both (R = H and CH 3 ) cases, the vibrational spectra are very simple, consisting entirely of sharp features readily assigned to fundamentals anticipated by their harmonic spectra.