Chromophore-containing quench agents 2 and 3 enable quantitative active site counting and determination of the mass distribution of active catalyst polymeryls by refractive index (RI) and UV detected gel permeation chromatography (GPC) for the polymerization of 1-hexene catalyzed by (EBI)ZrMe 2 /B(C 6 F 5 ) 3 . Time evolution of catalyst speciation data and the time profiles of monomer consumption, end-group generation, and bulk molecular weight distribution data have been analyzed by kinetic modeling to determine rate constants for initiation by insertion of hexene into a Zr−Me bond (k i ), propagation (k p ), chain transfer to form vinylidene (k 1,2 ) and vinylene (k 2,1 ) end groups, and reinitiation from a Zr−H bond (k r ). Unlike previous models that assumed fast catalyst reinitiation, this analysis reveals that k r is considerably slower than k p ; catalyst speciation data are critical to making this distinction. This study demonstrates that chromophore quench-labeling with 2 and 3 enables rapid, quantitative analysis of detailed kinetic models for catalytic olefin polymerization reactions using GPC with UV and RI detectors.