Competition experiments have been performed to determine the relative reactivities of substituted bromobenzenes and of different arylzinc reagents in the [Pd(PPh3)4]-catalyzed Negishi cross-coupling reaction in THF at 258C. The crosscoupling reactions are accelerated by electron acceptors in the bromobenzenes, the effect of which increases in the order ortho <meta <para. On the other hand, electron acceptors in the arylzinc halides diminish the reaction rates. Hammett correlations show that substituent variations in the bromobenzenes (1=+2.5) have a larger effect than substituent variations in the arylzinc halides (1=-0.98).