The characteristics of reactions in the monomer phase, aqueous phase, and the interface of monomer/aqueous phase of soap-free emulsion polymerization of 4-vinylpyridine (4VP) and styrene (St) system were evaluated by using the different types of initiators, addition of organic solvents, and changes of agitation rate. The oil-soluble initiators 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN), benzoyl peroxide (BPO), and 2,2'-azobis(isobutyronitrile) (AIBN), the organic water-soluble initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (V50), and the inorganic water-soluble initiator, potassium persulfate (KPS), were used. H-1 NMR and scanning electronic microscopy (SEM) were employed for the characterizations. As a result, the mechanism of interfacial particle formation was supported. Namely, the minimonomer droplets were generated by the disturbance in the interface of monomer/aqueous phase due to the agitation. The minimonomer droplets were stabilized by the adsorption of surface-active oligomer generated by the reactions in both the aqueous phase and the interface. The monomer transfer from the bulk monomer phase to the growing particles was via the coalescence of minimonomer droplets with particles. The role of reaction in the aqueous phase was proposed to just provide the surface-active oligomer for the stabilization of particles. The rapid reaction in the aqueous phase due to the high concentration of hydrophilic monomer produced longer hydrophilic chains and led to the coagulation of particles by a bridging-coagulation effect. On the basis of this mechanism, the coagulum-free stable latices with high monomer conversion were prepared by using KPS and AIBN and, theoretically, can be prepared by using any type of initiators at a high level of solid content and feed ratio of hydrophilic monomer.