Diosmacyclobutanes Os-2(CO)(8)(olefin) undergo facile exchange reactions with olefins and acetylenes. An associative mechanism is excluded by the observation of saturation kinetics as the concentration of the entering olefin (butyl acrylate) is increased. Multivariate analysis of the rate as a function of [C2H4] (determined from solubility measurements in decane/butyl acrylate mixtures) and [butyl acrylate] suggests a stepwise mechanism in which the departing olefin migrates to a terminal position and then undergoes associative exchange with the entering olefin. The straightforward dissociative exchange of the olefin ligands in Os(CO)(4)(olefin) shows saturation at much lower concentrations of entering olefin.