The effect of the substrate structure and the catalytic system on the rate of carbonylation of aryl halides in the presence of modified cobalt carbonyls was studied. Electron-acceptor substituents in the aryl halides favor faster carbonylation. The reaction mechanism was examined, and evidence was obtained for aryl halide activation by the S_RN1 mechanism. The activity order of cocatalysts is considered, and new catalytic systems were proposed.