The matrix isolation technique, combined with infrared spectroscopy and theoretical calculations, has been employed to explore the reaction chemistry of the VCl 4 /(CH 3 ) 2 CO system. Initial twin jet deposition led to formation of an isolated 1:1 complex between the subunits. This complex was characterized by strongly shifted CO and V–Cl stretching modes, as well as by lesser perturbations to additional modes of acetone. Theoretical calculations determined a binding energy for the complex of ΔE 0 298 =−4.3kcal/mol at the UB3LYP/6-31++G(d,2p) level of theory. Calculated infrared spectra, including deuterium shifts, were overall in good agreement with the experimental spectra. Bands due to the complex grew upon annealing, as well as upon irradiation prior to annealing. Matrices irradiated after annealing showed no change. This result differs markedly from the chemistry of the OVCl 3 /(CH 3 ) 2 CO system, and demonstrates the role of the oxo group in the reaction chemistry.