The alkylaluminum-complexed zirconocene trihydride cation [(SBI)Zr(mu-H)(3)((AlBu2)-Bu-i)(2)](+), which is obtained by reaction of (SBI)ZrCl2 with [Ph3C][B(C6F5)(4)] and excess HAI(i)8u(2) in toluene solution, catalyzes the formation of isotactic polypropene when exposed to propene at -30 degrees C. This cation remains the sole observable species in catalyst systems free of AlMe compounds. In the presence of AlMe3, however, exposure to propene causes the trihydride cation to be completely converted, under concurrent consumption of all hydride species by propene hydroalumination, to the doubly Me-bridged cation [(SBI)Zr(mu-Me)(2)AlMe2](+). The latter then becomes the resting state for further propene polymerization, which produces, by chain transfer to Al, mainly AlMe2-capped isotactic polypropene.