The ring-closure reaction of M(CO)(5)L-L (M=W, L-L=bipyridine) to produce M(CO)(4)(L-L) in nonpolar supercritical (sc) CO2/benzene mixtures was selected to investigate the repulsive (or intrinsic) part of the activation volume associated with this reaction. The activation volumes over the range between 7.5 and 24 MPa at different temperatures were higher than those in liquid benzene and liquid CO2/benzene, but much lower than those in pure sc CO2. The UV/vis spectral studies of this reaction showed that the addition of small amounts of cosolvent can increase the solubility of the ring-closure species. The kinetic traces recorded for the reaction also demonstrated this solubility enhancement.