A theoretical study of the catalyzed carbonylation process on the model system CH3-Ni(CO)2Cl reacting with a CO molecule has been carried out using a DFT approach. It has been found that two different reaction channels lead to the carbonylation products (acyl complexes). Along one reaction channel the carbonyl insertion takes place on five-coordinated nickel complexes, while the other reaction channel only involves the formation of four-coordinated complexes. The two reaction pathways require the overcoming of similar energy barriers for the insertion process (the barriers for the rate-determining step are 4.36 and 6.83 kcal mol-1 in the two cases, respectively). Even if for the model system considered here one reaction channel is slightly more convenient than the other, the computational results suggest that for the real system the two reaction paths can become highly competitive and their relative importance can change depending on the experimental conditions.