To elucidate the mechanism of the cobalt-catalyzed ACT reaction, we conducted a series of experimental and computational studies. The introduction of a radical scavenger, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), significantly inhibited the reaction (Fig. ). A radical-clock reaction led to a ring-opening alkene product (2au) (Figure ). These radical trap and radical clock experiments suggested a radical-mediated mechanism. Crossover experiments with substrates 1h and 1ag resulted in exclusive formation of non-crossover products 2h and 2ag, indicating an intramolecular acyloxy shift mechanism (Figure ). Further studies using deuterated substrates revealed a deuterium-shifted product 2aw (Figure ), and crossover experiments with deuterated and non-deuterated substrates produced crossover products 2a-d, supporting an intermolecular MHAT process (Figure ). Kinetic isotope effect (KIE) studies showed a KIE value of 2.9, implying a primary kinetic isotope effect (Figure ). Additionally, 18 O-labeling experiments with substrate 18 O-1h yielded products 18 O-2h-1 and 18 O-2h-2 in a 5.4:1 ratio, suggesting both [1,2]and [2,3]-acyloxy group shift pathways are operational (Figure ). Density functional theory (DFT) analysis showed that the transfer of a hydrogen atom from [Co III ]-H to the γ position of allyl carboxylate (TS-1a) is 5.10 kcal/mol more favorable than to the β position (TS-1b), resulting in the formation of radical intermediate II (Figure ). The transition state energy for the 1,2-RaM in the 3-membered ring ([1,2]-TS) is 1.5 kcal/mol lower than in the 5-membered ring ( [2,3]-TS), indicating a preference for the [1,2] pathway, resemble with experimental selectivity (Figure ). Additionally, the abstraction of a hydrogen atom from radical intermediate III by the [Co II ] catalyst is the rate-determining step (TS-2a), with an energy barrier of 13.7 kcal/mol, consistent with the KIE studies (Figure ). This step is more favorable than abstracting the hydrogen atom from the β-carbon (TS-2b, ∆∆G ‡ = 11.4 kcal/mol), explaining the regioselective formation of product 2.