A model which describes the mass transfer and interfacial equilibrium effects during swelling of monomer droplets and polymer particles, is presented. These phenomena have applications in diffusional stability of oil-in-water emulsions, which is critical to the production and polymerization of miniemulsions. The model includes an improved mass transfer coefficient which shows an appropriate dependence on the particle/droplet size. In addition an elastic energy term has been incorporated into the Gibbs free energy of formation of the partcles/droplets to account for elastic deformation during swelling. The model is used to show that polymer may be used as a cosurfactant in miniemulsion polymerization. Such emulsions are not thermodynamically stable, but can be thought to be kinetically stable in the sense that Ostwald ripening of the small monomer droplets is sufficiently retarded so as to allow nucleation and polymerization of the droplets before significant diffusional degradation of the droplet population has taken place.