A vapor phase monomer addition method was used to measure the monomer transfer rate into latex particles. The classical mathematical model for the diffusion process was used to calculate the overall mass transfer coefficients of monomer between the vapor and the water phases, and between the water and the polymer particle phases. At the initial stage of the monomer transfer into the glassy polystyrene latex particles at 60 degrees C, the rate-determining step was the monomer transfer inside the polymer. As more monomer transferred into the latex particles and the glass transition temperature of the swollen latex particles decreased below the experimental temperature, the rate-determining step became the monomer transfer across the interfacial surfactant layer. This transition was not observed with poly(vinyl acetate) latex, which was rubbery at the experimental temperature. The diffusion coefficients calculated were in the order of 10(-13) to 10(-14) cm(2)/s. These low values correspond to molecular diffusion of small molecules through a solid or glassy membrane. This was attributed to the resistance of the adsorbed surfactant layer. The mass transfer rates into latex particles stabilized with ionic, nonionic surfactants, and a water-soluble polymer were compared. In the case of the ionic surfactant, sodium dodecyl sulfate, when a large amount of electrolyte was added, the mass transfer rate decreased compared to the electrolyte addition free case. This was attributed to the formation of a "condensed" layer of adsorbed surfactant. Adsorbed nonionic surfactant or water-soluble polymer showed larger resistance to monomer transport compared to the ionic surfactant layer.