The hydration of phenylacetylene in the presence of the complex mer, trans-(PNP)RuCl2(PPh(3)) in THF at 60 degrees C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac, cis-(PNP)RuCl2(CO) and toluene [PNP=CH3CH2CH2N(CH(2)CH(2)PPh(2))(2)]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to sigma-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized : the vinylidene fac,cis-(PNP)RuCl2{C=C(H)Ph}, the (aquo)(sigma-alkynyl) complex fac-(PNP)RuCl(C=CPh)(OH2), and the (benzyl)carbonyl mer-(PNP)RuCl(eta(1)-CH(2)Ph)(CO). Other intermediates such as the sigma-acyl mer-(PNP)RuCl(eta(1)-COCH(2)Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac, cis-(PNP)RuCl2{C(NC5H10)(CH(2)Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction.