The adduct formation between CD3CN and the Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Sr2+, Mn2+, Fe2+, Cu+, Cu2+, Zn2+, Au+, Ag+, Cd2+, Hg2+and Tl3+cations has been investigated by FTIR spectroscopy. The v2vibrational mode, mainly consisting of the CN stretching vibration, was studied. Upon adduct formation with a Lewis acid this band shifted to higher frequencies relative to that of pure CD3CN. In spite of the widely varying electronic configurations of the cations studied, this frequency shift was found to' be approximately proportional to the polarizing power of the cation, measured as the charge to squared radius ratio, suggesting that the cation-acetonitrile interaction is of mainly electrostatic nature. No evidence of π back-donation was detected.