1,3-Butadiene, 4-methyl-1,3-pentadiene and styrene were polymerized with dicyclopentadienyltitanium dichloride/methylaluminoxane (Cp(2)TiCl(2)/MAO) and dicyclopentadienyltitanium chloride/MAO (Cp(2)TiCl/MAO). These systems are less active than cyclopentadienyltitanium trichloride/MAO (CpTiCl(3)/MAO), but show the same stereospecificity as the latter; they give predominantly cis-1,-polybutadiene, 1,2-syndiotactic poly(4-methyl-1,3-pentadiene) and syndiotactic polystyrene. Cp(2)TiCl/MAO is much more active than Cp(2)TiCl(2)/MAO; this is probably due to the fact that in the reaction of Cp(2)TiCl(2) with MAO, only a small amount of Ti(IV) is reduced to Ti(III), which is the active species in the polymerization of styrene and 1,3-dienes. An interpretation of the structure of the active species in Cp(2)TiCl/MAO is reported.