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We developed a novel fluorescence labeling technique for quantification of surface densities of atom transfer radical polymerization (ATRP) initiators on polymer particles. The cationic P(St-CPEM-C 4 DMAEMA) and anionic P(St-CPEM) polymer latex particles carrying ATRP-initiating chlorine groups were prepared by emulsifier-free emulsion polymerization of styrene (St), 2-(2-chloropropionyloxy)ethyl methacrylate (CPEM), and N-n-butyl-N,N-dimethyl-N-(2-methacryloyloxy)ethylammonium bromide (C 4 DMAEMA). ATRP initiators on the surface of polymer particles were converted into azide groups by sodium azide, followed by fluorescent labeling with 5-(N,Ndimethylamino)-N 0 -(prop-2-yn-1-yl)naphthalene-1-sulfonamide (Dansyl-alkyne) by copper-catalyzed azide-alkyne cycloaddition (CuAAC). The reaction time required for both azidation of ATRP-initiating groups and successive fluorescence labeling of azide groups with Dansyl-alkyne by CuAAC were investigated in detail by FTIR and fluorescence spectral measurement, respectively. The ATRP initiator densities on the cationic P(St-CPEM-C 4 DMAEMA) and anionic P(St-CPEM) particle surfaces were estimated to be 0.21 and 0.15 molecules nm 22 , respectively, which gave close agreement with values previously determined by a conductometric titration method. The fluorescence labeling through click chemistry proposed herein is a versatile technique to quantify the surface ATRP initiator density both on anionic and cationic polymer particles.