The combined kineQc and computaQonal approach allows a revised mechanisQc model for palladiumcatalysed formylaQon to be proposed. Data are inconsistent with oxidaQve addiQon being turnover-limiQng, as had been previously suggested, but rather support a turnover-limiQng sequence involving a combinaQon of a reversible migratory inserQon step and dihydrogen acQvaQon step. This sequence not only explains the kineQc behavior, it also predicts the divergent behavior of electron-rich and electron-poor aryl bromides through contribuQon of the migratory inserQon equilibrium to the overall rate. Hence for electron-rich substrate this equilibrium is both faster and more displaced toward the products leading to an overall faster rate and fewer side-reacQons, for electron-poor substrates the equilibrium is slower to form and more displaced toward starQng materials, reducing the overall rate and opening up hydrodebrominaQon side reacQons. We believe our findings will have important implicaQons in the development of both catalysts and opQmum process condiQons for the formylaQon of aryl halides with CO:H2 mixtures.