The mechanism of the formation of phenol from PhI and CsOH catalysed by copper(I) ligated to the 1,3- diketonate ket'− generated from 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD) has been investigated by DFT calculations associated with experimental techniques: cyclic voltammetry, 1H NMR, and ESI-MS. Weak halogen bonding between the negatively charged O atom of [(ket')CuI-OH]− and PhI leads to an oxi- dative addition that gives (ket')CuIII(Ph)-OH. The latter undergoes a faster reductive elimination that deli vers (ket')CuI(PhOH) from which PhOH is released. PhOPh is formed in the presence of an extra base Cs2CO3. The two catalytic cycles of formation of PhOH or PhOPh are branched at the level of (ket') CuI(PhOH) that can either afford PhOH in the presence of CsOH or be deprotonated by Cs2CO3 to gener- ate [(ket')CuI-OPh]−. The oxidative addition of [(ket')CuI-OPh]− to PhI leads to (ket')CuIII(Ph)-OPh involved in a faster reductive elimination that delivers PhOPh and the CuI catalyst.