Homogeneous nucleation is fundamentally important in emulsion polymerization. A molecular theory is proposed to quantify primary particle formation in the process. The proposed model divides a polymerization system into three portions: the domains formed by growing radicals, their surrounding aqueous solution, and dispersed monomer droplets. In general, the total free energy of the domains is contributed from the mixing among the molecules including monomer and water, the elasticity of oligomeric radical chains, and the transferable free energy of electrolytes; while that of the outer solution is from the mixing of monomer and water molecules and the transferable energy of electrolytes. Application of this theory to vinyl acetate emulsion polymerization has shown that the critical degree of polymerization (j(cri)) predicted is in a good agreement with the value derived from experimental data reported in literature. Furthermore, this model can also estimate the concentrations of VAc and water in the domain at the degrees of polymerization around j(cri). (c) 2008 Elsevier Inc. All rights reserved.