Transmetalation is the rate-limiting step! The transmetalation between arylzinc reagents and (ArNiR)-R-II was confirmed as the rate-limiting step in the nickel-catalyzed oxidative coupling reactions. It was proved to be an excellent model allowing the first quantitative measurement of the kinetic rate constants of transmetalation from a live catalytic system. Rate constants from 0.04 to 0.31 M-1 s(-1) were obtained for different arylzinc reagents under the conditions, and the activation enthalpy Delta H double dagger was 14.6 kcal/mol for PhZnCl. The substituent effect on the transmetalation was also gained for the first time from the catalytic reaction.