Single-site polymerization catalysts enable exquisite control over alkene polymerization reactions to produce new materials with unique properties. Knowledge of catalyst speciation and fundamental kinetics are essential for full mechanistic understanding of zirconocene-catalyzed alkene polymerization. Currently the effect of activators on fundamental polymerization steps is not understood. Progress in understanding activator effects requires determination of fundamental kinetics for zirconocene catalysts with noncoordinating anions such as [B(C6F5)4]-. Kinetic NMR studies at low temperature demonstrate a very fast propagation rate for 1-hexene polymerization catalyzed by [(SBI)Zr(CH2SiMe3)][B(C6F5)4] [where SBI is rac-Me2Si(indenyl)2] with complete consumption of 1-hexene before the first NMR spectrum. Surprisingly, the first NMR spectrum reveals, aside from uninitiated catalyst, Zr-allyls as the sole catalyst-containing species. These Zr-allyls, which exist in two diastereomeric forms, have been characterized by physical and chemical methods. The mechanism of Zr-allyl formation was probed with a trapping experiment, leading us to favor a mechanism in which Zr-polymeryl undergoes beta-H transfer to metal without dissociation of coordinated alkene followed by sigma-bond metathesis to form H2 and Zr-allyl. Zr-allyl species undergo slow reactions with alkene but react rapidly with H2 to form hydrogenation products.