EtCN partially displaces coordinated carbon monoxide from cis-PtCl2(CO)2 giving an equilibrium mixture of the two geometrical isomers of PtCl2(CO)(NCEt), together with unreacted cis-PtCl2(CO)2, as monitored by IR and NMR measurements. The equilibrium has also been studied starting from PtCl2(NCEt)2, through displacement of coordinated EtCN by CO. The equilibrium constant of the reaction between PtCl2(CO)(NCEt) [cis + trans] and CO to produce cis-PtCl2(CO)2 (48 ± 6, corresponding to ΔG0 = -9.5 ± 0.3 kJ mol-1) has been measured at 23.4 °C, in the presence of SnCl2 as catalyst, the uncatalysed reaction being exceedingly slow. With an appropriate control of the CO partial pressure, PtCl2(CO)(NCEt) was obtained in a nearly quantitative yield either from cis-PtCl2(CO)2 + EtCN or from PtCl2(NCEt)2 + CO. The molecular and crystal structure of cis-PtCl2(CO)(NCEt) has been solved by X-ray diffractometry.