Investigation of the mechanism of the carbonylation of aryl halides in a catalytic system consisting of cobalt carbonyl, terminal olefin oxide, and base showed that this catalytic system activates aryl halides via a radical anion mechanism. The kinetic regularities of the process together with IR spectral data suggest reversible formation of the active catalytic complex under the reaction condition. The complex is a cyclic metal lactone, and it can form not only from olefin oxide but also from its methanolysis product, the corresponding glycol monomethyl ether.