The mechanism of the unique suspension polymerization of uniform monomer droplets, without coalescence and breakup during the polymerization, was investigated using styrene (S) as a monomer mixed with water-insoluble hexadecane (KD). The glass membrane (Shirasu Porous Glass, SPG) emulsification technique was employed for the preparation of uniform droplets. Depending an the pore sizes of the SPG membranes (1.0, 1.4, and 2.9 mu m), polymer particles of an average diameter ranging from 5.6 to 20.9 mu m were obtained with the coefficient of variation (CV) being close to 10%. The role of HD was to prevent the degradation of the droplets by the molecular diffusion process. Sodium nitrite was added in the aqueous phase to kill the radicals desorbed from the droplets (polymer particles), thereby suppressing the secondary nucleation of smaller particles. Each droplet behaved as an isolated locus of polymerization. With the presence of HD, the initial polymerization rate was proportional to 0.24th power of the benzoil peroxide (BPO) concentration. This peculiar behavior as compared with the ordinary suspension polymerization was explained by introducing the assumption that each droplet was composed of isolated compartments (cells) in which active polymeric radicals were dissolved in an S-rich phase and surrounded by a rather incompatible S/HD (continuous) phase. The average number of radicals in the droplet increased initially due to the separate existence of polymeric radicals in compartments. As the polymerization progressed, the HD-rich phase gradually separated, eventually forming macrodomains, which were visible by an optical microscope. The phase separation allowed polystyrene chains to dissolve in a more favorable S phase, and the homogeneous bulk polymerization kinetics took over, resulting in a gradual decrease of the average number of radicals in the droplet until the increase of viscosity induced the gel effect. When no HD was present in the droplets, the polymerization proceeded in accordance with the bulk mechanism except for the initial retardation by the entry of inhibiting radicals generated from sodium nitrite in the aqueous phase.