A systematic theoretical study on reaction mechanisms for copper(I)-catalyzed C–O coupling of phenols with aryl bromides is reported herein. Through evaluation of the relative concentrations of possible copper species in reaction solution and reactivity study of these copper species with aryl halides in the context of several commonly proposed mechanisms for copper(I)-catalyzed Ullmann reactions, we propose that the active copper catalyst should be a neutral (L)Cu(I)–OAr (L denotes an ancillary ligand; OAr denotes an aryloxide ligand) species in nonpolar solvent and Cu(OAr)₂⁻ anion in highly polar solvent. In the reaction solution, these two kinds of copper species should be in equilibrium, the direction of which is highly dependent on the polarity of the solvent. For both kinds of copper species, a halogen atom transfer mechanism is favored where an initial halogen atom transfer from phenyl bromide to the Cu(I) center occurs, resulting in the formation of Cu(II)(OAr)(Br) and a phenyl radical. Subsequent rapid attack of this phenyl radical to the aryloxide ligand bound to copper(II) would yield the coupling product and Cu(I)(Br) species, which can be readily converted to the active Cu(I)–OAr species in the presence of phenols and base. Other mechanisms such as oxidative addition, single electron transfer and σ-bond metathesis mechanisms all possess activation barriers which are too high, rendering them kinetically unfavorable. Electronic effects on phenol rings reveal that electron-donating substituents accelerate the coupling of (phen)Cu(I)(OAr) with aryl halides whereas electron-withdrawing substituents lead to much higher activation barriers, which is consistent with experimental findings and thus lends further support for such a halogen atom transfer mechanism. In view of the widely accepted oxidative addition/reductive elimination mechanism for analogous copper(I)-catalyzed coupling of N-nucleophiles with aryl halides, our results here highlight that the reaction mechanism of copper(I)-catalyzed Ullmann reactions is highly dependent on the nature of the nucleophile and different kinds of nucleophiles can be involved in different mechanisms.