Reactions of vanadium dioxide molecules with acetylene have been studied by matrix isolation infrared spectroscopy. Reaction intermediates and products are identified on the basis of isotopic substitutions as well as density functional frequency calculations. Ground state vanadium dioxide molecule reacts with acetylene in forming the side-on-bonded VO2(eta(2)-C2H2) and VO2(eta(2)-C2H2)(2) complexes spontaneously on annealing in solid neon. The VO2(eta(2)-C2H2) complex is characterized to have a B-2(2) ground state with C-2v symmetry, whereas the VO2(eta(2)-C2H2)(2) complex has a (2)A ground state with C-2 symmetry. The VO2(eta(2)-C2H2) and VO2(eta(2)-C2H2)(2) complexes are photosensitive. The VO2(eta(2)-C2H2) complex rearranges to the OV(OH)CCH molecule upon UV-vis light excitation.