A theoretical investigation at the DFT(B3LYP) level on the carbonylation reaction of allyl bromide catalyzed by nickel tetra-carbonyl Ni(CO)4 is discussed. The computational results show the following: (i) Three main steps characterize the catalytic cycle: (a) an oxidative addition step, (b) a carbonylation step, and (c) a reductive elimination step where the acyl product is obtained and the catalyst is regenerated. (ii) Both Ni(CO)3 and Ni(CO)4 complexes can behave as "active" catalytic species. (iii) The oxidative addition leads to the formation of either η 3 or η1-allyl nickel complexes, which are involved in a fast equilibrium. (iv) The carbonylation occurs much more easily on the η1 than on the η3 intermediates.