Emulsifier-free emulsion polymerization of styrene was performed in the presence of small amount of methacrylic and itaconic acids as carboxylic acid monomers and potassium persulfate as an initiator at 70 degrees C to prepare monodisperse polymer particles. Diameter of monomer swollen polymer particles (d(pswol)) was measured by dynamic light scattering for samples taken from the reaction mixture during the Intervals II and III of the emulsion polymerization. Graphically treatment of d(pswol) versus conversion data allowed us for the first time to directly determine the critical monomer conversion (x(c)), from which constant monomer concentration in the polymer particles (C-MP) during the Interval II was then calculated. x(c), and C-MP were obtained to be 0.379 and 5.68, respectively. C-MP value is in good agreement with that obtained by centrifugation method and those reported in the literature for the similar system. Attempts were also made to evaluate the average number of growing chain per particle ((n) over bar) during the Interval II of emulsion polymerization of styrene. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1055-1063, 2009