Polymerization in vesicles is a novel type of polymerization in heterogeneous media, leading to parachute-like vesicle-polymer hybrid morphologies. To explore the kinetics of vesicle polymerizations and to learn more about the actual locus of polymerization we applied the pulsed-laser polymerization (PLP) technique to the polymerization of styrene in small and large unilamellar dioctadecyldimethylammonium bromide (DODAB) vesicles. The analysis of the molecular weight distribution (MWD) allows primarily the determination of the monomer concentration at the site of polymerization if the propagation rate coefficient, kp, of the applied monomer is known. Relatively high monomer concentrations (between 2.6 and 4.5 mol/L) were found at the locus of polymerization at low conversions. PLP experiments as a function of overall monomer concentration, temperature, and vesicle size gave insight into the monomer partitioning between vesicle and polymerization site. With respect to the mechanism of polymerization in vesicles, the observations seem to indicate that polymerization occurs in a small polymer nucleus where high monomer concentrations are encountered.