Reaction kinetics and mechanistic studies for ethylene-internal alkyne metathesis promoted by the phosphine-free initiator Ru1 (Piers's catalyst) is described. The kinetic order of reactants and catalyst was determined. The effect of ethylene was studied at different solution concentrations using ethylene gas mixtures applied at constant pressure. Unlike earlier studies with the second-generation Grubbs complex, ethylene was found to show an inverse first-order rate dependence. Under catalytic conditions, a ruthenacyclobutane intermediate was observed by proton NMR spectroscopy at low temperature. Combined with the kinetic study, these data suggest a catalytic cycle involving a reactive LnRu=CH2 species in equilibrium with ethylene to form a ruthenacyclobutane, a catalyst resting state. Rates were determined for a variety of internal alkynes of varying substitution. Also, at low ethylene pressures, preparative syntheses of several 2,3-disubstituted 1,3-butadienes were achieved. Using the kinetic method, several phosphine-free inhibitors were examined for their ability to promote ethylene alkyne metathesis and to guide selection of the optimal catalyst.