The decomposition of a series of ruthenium metathesis catalysts has been examined using methylidene species as model complexes. All of the phosphine-containing methylidene complexes decomposed to generate methylphosphonium salts, and their decomposition routes followed first-order kinetics. The formation of these salts in high conversion, coupled with the observed kinetic behavior for this reaction, suggests that the major decomposition pathway involves nucleophilic attack of a dissociated phosphine on the methylidene carbon. This mechanism also is consistent with decomposition observed in the presence of ethylene as a model olefin substrate. The decomposition of phosphine-free catalyst (H(2)IMes)(Cl)(2)RuCH(2-C6H4-O-i-Pr) (H(2)IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) with ethylene was found to generate unidentified ruthenium hydride species. The novel ruthenium complex (H(2)IMes)(pyridine)(3)(Cl)(2)Ru, which was generated during the synthetic attempts to prepare the highly unstable pyridine-based methylidene complex (H(2)IMes)(pyridine)(2)(Cl)(2)RuCH2, is also reported.