Reactions of PhC=CH (la) and p-CH3C6H4C=CH (1b) with [Ir(RCN)(CO)(PPh(3))(2)]ClO4, 2 (R = Ph (a), CH3 (b), CH=CHPh (c)), give [Ir(H)(-C=CR’)(RCN)(CO)(PPh(3))(2)]ClO4, 3 (R’ = Ph, R = Ph (a), CH3 (b), CH=CHPh (c); R’ = P-CH3C6H4, R = Ph (d)), where the hydride is cis both to two PPh(3) groups and trans to RCN. The oxidative addition of 1a to 2a,b is first order both in 1a and 2a,b and depressed by the presence of RCN added. The suggested reaction pathway involves the initial replacement of RCN of 2a,b by 1a to give [Ir(PhC=CH).(CO)(PPh(3))(2)]ClO4, which undergoes an intramolecular oxidative cleavage of the coordinated 1a to give [Ir(H)(-C=CPh)(CO) (PPh(3))(2)]ClO4, to which RCN is added back to give the stable complexes 3a,b. Kinetic data obtained for alkyne hydrogen deuterated phenylacetylene, PhC=CD (1a-d), support the suggested mechanism.