Monosubstituted acyclic (E)-1,3-dienes undergo efficient hydrovinylation giving (Z)-3-alkylhexa-1,4-dienes upon treatment with catalytic amounts of bidentate phosphine-CoCl2 complexes {[P similar to](CoCl2)} and Me3Al in an atmosphere of ethylene. The regioselectivity Utile reaction (i.e., 1,4- or 1,2-addition) depends on the nature of the ligand and temperature at which the reaction is carried out. Complexes derived from (RR)-DIOP and (SS)-BDDP at 45 C give very high enantioselectivities for several prototypical 1,3-dienes. In sharp contrast to the corresponding Ni(II)-catalyzed hydrovinylation, I-aryl-substituted 1,3-dienes give almost exclusively achiral linear 1,4-addition products, unless the 2-position is also substituted. Solid-state structures of the precatalysts are presented.