The kinetics of acyclic diene metathesis (ADMET) promoted by (PCy3)(2)Cl2RuCHPh (2), and the more recently developed catalyst 3, which contains the 1,3-bis(mesityl)imidazolidene-2-ylidene (SIMes) ligand, were measured and compared. Complex 3 is clearly the more active catalyst at temperatures of 45-75 degreesC, while complex 2 is actually the faster catalyst at 30 degreesC. Although no induction period was observed for complex 2, a conspicuous delay in activity was observed at the onset of ADMET with complex 3 at all temperatures studied. We propose that this behavior is due to the rates of phosphine dissociation for these catalysts, as is consistent with new mechanistic data in the context of other olefin metathesis reactions.