Cobalt-phosphine and cobalt-carbene catalysts have been developed for the hydroarylation of styrenes via chelation- assisted C-H bond activation, to afford branched and linear addition products, respectively, in a highly regioselective fashion. Deuterium-labeling experiments suggested a mechanism involv- ing reversible C-H bond cleavage and olefin insertion steps and reductive elimination as the rate- and regioselectivity-determining step.