Stopped-flow NMR measurements suitable for determination of reaction kinetics on time scales of 100 ms or longer have been achieved by adaptation of a commercial NMR flow probe with a high-efficiency mixer and drive system. Studies of metallocene-catalyzed alkene polymerization at room temperature have been complicated by high rates, imprecise knowledge of the distribution of different catalyst species with time, and the high sensitivity of the catalysts to low concentrations of impurities. Application of the stopped-flow NMR method to the study of the kinetics of 1-hexene polymerization in the presence of (EBI)ZrMe[MeB(C6F5)(3)] demonstrates that NMR spectroscopy provides an efficient method for direct and simultaneous measurement of substrate consumption and catalyst speciation as a function of time. Kinetic modeling of the catalyst and substrate concentration time courses reveal efficient determination of initiation, propagation, and termination rate constants. As first suggested by Collins and co-workers (Polyhedron 2005, 24, 1234-1249), a kinetic model in which Zr-HB(C6F5)(3) forms rapidly upon beta-hydride elimination but reacts relatively slowly with alkene to reinitiate chain growth is supported by these data.