The electrochemical reduction of CoBr2 in acetonitrile+pyridine (v/v=9:1) has been investigated. In this solvent mixture, the disproportionation reaction rate of the electrogenerated Co(I) is about twice as slow as in dimethylformamide+pyridine (9:1) and the reaction between Co(I) and arylhalides, ArX, also is slowed down by the same factor. We show that the use of vinyl acetate (vinyl-OAc) or methyl vinyl ketone (MVK) as additives in acetonitrile+pyridine increases the stability of Co(I). Thus, in the presence of vinyl-OAc the disproportionation reaction rate of Co(I) is divided by seven whereas the rate of the oxidative addition of Co(I) with aryl halides is divided only by two. Preparative-scale electrolyses of arylzinc compounds confirm this result since the quantity of catalyst, CoBr2, can be divided by four as compared to the amount used in the absence of additives. In the two cases (presence of vinyl-OAc or MVK) the presence of pyridine is required to stabilize Co(I). In these reactions, the rate constants for both chemical reactions (disproportionation reaction and oxidative addition) have been determined and the stabilization of Co(I) is discussed in terms of formation of π-complexes with the additives.