A series of ruthenium olefin metathesis catalysts of the general structure (H(2)IMes)(PR3)(Cl)(2)-Ru = CHPh (H(2)IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) have been prepared; these complexes are readily accessible in two steps from commercially available (H(2)IMes)(PCy3)(Cl)(2)Ru = CHPh. Their phosphine dissociation rate constants (k(1)), relative rates of phosphine reassociation, and relative reaction rates in ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM) have been investigated. The rates of phosphine dissociation (initiation) from these complexes increase with decreasing phosphine donor strength. Complexes containing a triarylphosphine exhibit dramatically improved initiation relative to (H(2)IMes)(PCy3)(Cl)(2)Ru = CHPh. Conversely, phosphine reassociation shows no direct correlation with phosphine electronics. In general, increased phosphine dissociation leads to faster olefin metathesis reaction rates, which is of direct significance to both organic and polymer metathesis processes.