The kinetics of emulsifier-free emulsion copolymerization of styrene were investigated in the presence of two amino-containing monomers, aminoethyl methacrylate hydrochloride (AEMH) and vinyl benzyl amine hydrochloride (VBAH), using 2,2’-azobis (2-amidinopropane) dihydrochloride (V50) as a cationic initiator. At first the partition coefficient of AEMH and VBAH in a water-styrene mixture were measured, indicating a strong hydrophilicity for both monomers; kinetics of solution homopolymerization, as followed by H-1-NMR, in water provided high k(p)/k(t)(1/2) values at 4.0 and 1.75 (L mol(-1) s(-1))(1/2) for AEMH and VBAH, respectively. The two monomers were found to similarly affect the kinetics of emulsion copolymerization of styrene : the overall polymerization rate and particle number increased dramatically upon increasing the functional monomer concentration; the average number of radicals per particle ((n) over bar) was dependent upon the particle size, reaching a 0.5 value for particle size below 100 nm; and the molecular weight of polymer samples decreased with the functional monomer concentration, revealing the strong activity of VBAH and AEMH in the chain transfer. The polymerization mechanism in the presence of VBAH and AEMH was discussed, referring to the coagulation nucleation mechanism to explain the formation of a large particle number. Using the method proposed by Gilbert et al., transfer rate constants were determined for each monomer (around 10 L mol(-1) s(-1)) and styrene (4.5 10(-2) L mol(-1) s(-1)).