Alkenes (cyclohexene or 1-octene) and dienes (cyclo-octadiene, COD; or norbornadiene, NBD) react with cis-PtCl2 (CO) 2, leading, respectively, to partial and complete displacement of coordinated carbon monoxide. The reactions have been monitored by gas-volumetric methods and by IR and NMR spectra. Thermodynamic data were obtained by IR and UV spectroscopies.