In combinaQon, the kineQc data suggest that the precise mechanism of formylaQon of aryl bromides catalysed by [Pd(PAd2 n Bu)2] is substrate dependent. The Hammee analysis is consistent with a change in the turnover-limiQng sequence with bifurcaQon into two catalyQc regimes. Regime 1 can be defined for electron-deficient substrates with sp > 0, and regime 2 one for electron-rich substrates with sp < 0. The kineQc behavior of the catalyQc system across both regimes is, however, strikingly similar. In both regimes, 1 st order behavior in catalyst suggests that monomeric palladium intermediates are likely involved in the turnover-limiQng step. Catalyst speciaQon appears consistent across the Qmecourse, with no evidence for deacQvaQon or change in behavior across electron-rich and electron-poor substrates. For both electron-rich and electron-deficient substrate the order in aryl bromide is zero and there is no evidence of product inhibiQon. The observed zero-order behavior in substrate is not consistent with oxidaQve addiQon being part of the turnover-limiQng sequence, but rather suggests that it is a fast step that does not contribute significantly to the overall rate. It appears, however, that the base plays a role in the turnover-limiQng step, with clear parQal posiQve order close to 1 for the number of acQve basic sites. There is also a marked dependence of the reacQon rate on H2 parQal pressure, suggesQve of a role of dihydrogen in the turnover-limiQng step.