The four-coordinate ruthenium phosphonium alkylidenes 1-Cy and 1-iPr, differing in the substituent on the phosphorus center. were observed to decompose thermally in the presence of 1,1-dichloroethylene to produce [H3CPR3][Cl]. The Major ruthenium-containing product was a trichloro-bridged ruthenium dimer that incorporates the elements of the 1,1-dichloro-ethylene as a dichlorocarbene ligand and a styrenic vinyl group on the supporting NHC ligand. Spectroscopic, kinetic, and deuterium-labeling experiments probed the mechanism of this process, which involves a rate-limiting C-H activation of an NHC mesityl ortho methyl group. These studies provide insight into intrinsic decomposition processes of active Grubbs type olefin metathesis catalysts, pointing, the way to new catalyst design directions.