The first systematic TRIR study of the photolysis of M(CO)(6) in supercritical Ar, Kr, Xe, and CO2 permits the observation of M(CO)(5)L (M = Cr, Mo, and W; L = Ar (W only), Kr, Xe, and CO2). The second-order rate constants for the reaction of M(CO)(5)L with CO have been evaluated and the reactivity for each metal is Kr > Xe approximate to CO2. For M(CO)(5)Kr, M(CO)(5)Xe, or M(CO)(5)(CO2) the reactivity is Cr approximate to Mo > W. In supercritical Kr doped with either Xe or CO2, the M(CO)(5) moiety interacts with Xe or CO2 in preference to Kr. The effect of solvent density on the rate of the reaction of W(CO)(5)(CO2) with CO has been investigated. This is the first time that the density dependence of any dissociative reaction has been followed in this way in supercritical solution. Our observations demonstrate that the reaction of W(CO)(5)(CO2) with CO in scCO(2) is predominantly a dissociative process. The activation energies for the reaction of W(CO)(5)Xe and W(CO)(5)(CO2) with CO and the relative wavelength of the visible absorption maxima for Cr(CO)(5)Xe and Cr(CO)(5)(CO2) all indicate a similar strength of interaction for Xe and CO2 with the M(CO)(5) moiety.