Decomposition of the complexes (eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CR [R = c-C4H9, (CH2)(3)CH3, and CH(CH2-CH2CH3)(2)] in CHCl3 results in conversion of the carbyne ligand to a terminal olefin. The reaction is initiated by oxidation and occurs during photolysis in CHCl3 or upon slow diffusion of O-2 into the reaction mixtures. Corroborating evidence for initiation of the reaction by electron transfer has been obtained by conversion of the butyl carbyne ligand of (eta(5)-C5H5)(CO){P(OMe)(3)}Mo=C(CH2)(3)CH3 to 1-pentene upon electrochemical oxidation. Mechanistic studies were consistent with H-abstraction by the carbyne radical cation to yield a cationic carbene complex which forms the olefin in a H-shift process. INDO calculations on the carbyne radical cation [(eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CCH2CH3](.+) indicate stabilization upon bending the Mo=C-C angle from 180 degrees to 120 degrees. This change in geometry places spin density on the carbyne carbon in the radical cation although the initial oxidation occurs from an orbital that is primarily nonbonding metal d in character. The oxidized carbyne is thus able to function as a carbon-centered radical and abstract a hydrogen atom at the carbyne carbon. Although the olefin-forming reaction is general far alkyl carbynes with a hydrogen on C2, the tert-butyl carbyne (eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CC(CH3)(3) (2e) did not form an olefin upon oxidation. Instead, photolysis of 2e in CHCl3 yielded the dichloromolybdenum carbyne (eta(5)-C5H5)Cl-2{P(OMe)(3)}Mo=CC(CH3)(3) (5e) via a Cl-abstraction pathway.