The ability of a series of initiators with different water solubilities (lauroyl peroxide, benzoyl peroxide, and azobis(isobutyronitrile)) in stabilizing monomer droplets against degradation by molecular diffusion was investigated in the batch miniemulsion polymerization of styrene. Comparison of the evolution of the particle size distributions obtained in these experiments with those of a series of miniemulsion polymerizations where the stability of the monomer droplets was ensured by using hexadecane in addition to the oil-soluble initiators showed that only lauroyl peroxide was water-insoluble enough to stabilize the monomer droplets against degradation by molecular diffusion. The results were analyzed by a mathematical model.