The mechanism of boron-to-nickel transmetalation –the key step of the nickel-catalyzed Suzuki-Miyaura (S- M) coupling– was examined both experimentally and theoretically. Dinuclear μ hydroxo-bridged complexes formed by reaction of trans-[ArNi(PR3)2X] with hydroxide are not directly involved in transmetalation, but they rather act as a resting state for the catalyst. The base/boronic acid ratio is the crucial parameter, as it modulates the extent of formation of these dinu-clear species and thus tunes the catalytic activity. These findings explain some limitations encountered in practical appli-cations of nickel-catalyzed S-M couplings and suggest how to tailor the experimental conditions in order to overcome these difficulties.