In emulsion polymerization, monomer can be transferred into the polymer particles either by the diffusion from the aqueous phase or by the collision between the monomer droplets and the polymer particles. To investigate only the diffusional monomer transfer rate at the latex particle interface, a vapor phase addition method was developed. In this method, monomer is transferred from a separate reservoir to the aqueous phase, and then into the latex particles. Because there are no monomer droplets in the aqueous phase, the monomer transfer occurs only by diffusion. The vapor and the water phase resistances were minimized to the extent that the particle interfacial resistance dominated. Using this method, overall mass transfer coefficient of the particle-water interphase, K w p , can be calculated using conventional mass transfer expressions. For example, vinyl acetate monomer mass transfer coefficient, for monodisperse polystyrene latex particles stabilized with ionic surfactant, sodium lauryl sulfate, was determined. K w p was found to be independent of the agitator speed in the range of 400-500 rpm. The calculated value of K w p was in the order of 10 - 7 cm/min.