SYNOPSISSimple equations describing monomer partitioning in latices during intervals 2 and 3 in emulsion polymerization with any number of low to moderately water soluble monomers were derived from the extended Morton equation by making various assumptions. It appears that it is mainly the combinatorial entropy of mixing that governs the partitioning behavior, and that other contributions to the free energy of the monomers in the polymer particles are marginal. Experimental results with styrene, methyl methacrylate, and methyl acrylate confirm the validity of the assumptions. In interval 3 of emulsion polymerization the sum of all contributions to the free energy of the monomers in the particles other than the combinatorial entropy of mixing can be taken as a constant that is dependent only on the monomer composition in the particles and independent of the degree of swelling of the particles. The only parameters one needs to know to calculate the monomer concentrations in all phases with help of the derived equations, are the saturation concentrations of each monomer in the polymer particles, and the saturation concentrations of each monomer in the aqueous phase. The introduction of more than one monomer in emulsion polymerization systems is becoming more and more important in the manufacturing of emulsion polymers. This is simply a result of the recognition that a combination of several monomers in a polymer can result in a wide variety of properties. In quite a lot of cases these properties are superior to the ones obtained by homopolymerizing the single monomers. To make a better polymer by copolymerization one should be able to control the consumption rates of each monomer, i.e., one should be able to control composition drift, as this phenomenon often leads to inferior products. One of the prerequisites for controlling composition drift is knowledge about the thermodynamic partitioning of each monomer between the phases that constitute * To whom all correspondence should be addressed. an emulsion polymerization system (polymer, aqueous, and monomer phases). It is the thermodynamic partitioning that governs the concentration of each of the monomers in the polymer phase, where most of the polymerization takes place. In this article we present the full equations that describe the partitioning of any number of monomers during intervals 2 and 3 in emulsion polymerization. The equations are based on simplifications to the original Flory-Huggins lattice model and Morton's equation. We will give results of experiments with three monomers that confirm the validity of the derived equations. Journal of Polymer