A series of experiments were carried out on the soap-free emulsion polymerization mechanism of 4-vinyl pyridine (4VP)/styrene (St) and the solvent, ethyl acetate (EA), by using the cationic initiator, 2,2'-azobis(2-amidinopropane) 2HCl (V50). To investigate the mechanism of polymerization in detail, a particular quasi-static polymerization system-the reaction mixture in a sealed bottle allowed to stand without agitation during the whole polymerization period-was employed. The partition of monomers at ambient and solid content of the latex during the quasi-static polymerization in the presence of 8 wt % EA was measured by H-1-NMR, and the variation of particle size was observed by scanning electron microscopy (SEM). The solubility of 4VP and St in water decreased as the amount of EA increased, Meanwhile, most of the EA was partitioned in the oil phase, even in the cases where the charged amount of EA was much lower than the solubility of EA (similar to 8 wt %) in water at ambient. The solubility of EA in water was also affected by the composition of 4VP and St (i.e., increased as the fraction of 4VP increased, but decreased as the fraction of St increased). This observation, as well as the quantitative analysis of the quasi-static emulsion polymerization, indicated that the initial reaction in the soap-free emulsion polymerization was closely related to the disturbance of the interface of monomer/water phase, which triggered the generation of monomer droplets. Therefore, a new mechanism was proposed for the nucleation of soap-free emulsion polymerization, namely, the nucleation was performed in the droplets. The droplets absorbed the oligomeric. radicals generated in the aqueous phase to continue the polymerization, as well as to promote the colloidal stability of the droplets.