Homogeneous Cu/TEMPO catalyst systems (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) have emerged as some of the most versatile and practical catalysts for aerobic alcohol oxidation. Recently, we disclosed a (bpy)Cu^I/TEMPO/NMI catalyst system (NMI = N-methylimidazole) that exhibits fast rates and high selectivities, even with unactivated aliphatic alcohols. Here, we present a mechanistic investigation of this catalyst system, in which we compare the reactivity of benzylic and aliphatic alcohols. This work includes analysis of catalytic rates by gas-uptake and in situ IR kinetic methods and characterization of the catalyst speciation during the reaction by EPR and UV–visible spectroscopic methods. The data support a two-stage catalytic mechanism consisting of (1) “catalyst oxidation” in which Cu^I and TEMPO–H are oxidized by O₂ via a binuclear Cu₂O₂ intermediate and (2) “substrate oxidation” mediated by Cu^II and the nitroxyl radical of TEMPO via a Cu^II-alkoxide intermediate. Catalytic rate laws, kinetic isotope effects, and spectroscopic data show that reactions of benzylic and aliphatic alcohols have different turnover-limiting steps. Catalyst oxidation by O₂ is turnover limiting with benzylic alcohols, while numerous steps contribute to the turnover rate in the oxidation of aliphatic alcohols.