Several ruthenium-based olefin metathesis catalysts of the formula (PR3)(2)X2Ru=CHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis or the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined : X = I < Br < Cl and PR3 = PPh3 much less than (PPr2Ph)-Pr-i < PCy2Ph < (PPr3)-Pr-i < PCy3. Additional studies were conducted with the catalyst (PCy3)(2)-Cl2Ru=CH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways : the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.