The direct and single-step conversion of benzene, ethylene, and a Cu(II) oxidant to styrene using the Rh(I) catalyst ((FI)DAB)Rh(TFA)(eta(2)-C2H4) [(Fl)DAB = N,N '-bis(pentafluorophenyl)-2,3-dimethy1-1,4-diaza-1,3-butadiene; TFA = trifluoroacetate] has been reported to give quantitative yields (with Cu(II) as the limiting reagent) and selectivity combined with turnover numbers >800. This report details mechanistic studies of this catalytic process using a combined experimental and computational approach. Examining catalysis with the complex ((FI)DAB)Rh(OAc)(eta(2)-C2H4) shows that the reaction rate has a dependence on catalyst concentration between first-and half-order that varies with both temperature and ethylene concentration, a first-order dependence on ethylene concentration with saturation at higher concentrations of ethylene, and a zero-order dependence on the concentration of Cu(II) oxidant. The kinetic isotope effect was found to vary linearly with the order in ((FI)DAB)Rh(OAc)(eta(2)-C2H4), exhibiting no KIE when [Rh] was in the half-order regime, and a k(H)/k(D) value of 6.7(6) when [Rh] was in the first-order regime. From these combined experimental and computational studies, competing pathways, which involve all monomeric Rh intermediates and a binuclear Rh intermediate in the other case, are proposed.