Carboxylic acid monomers are commonly used at low concentrations as functional additives in emulsion polymerization. Being quite water-soluble, they partition between the aqueous and polymer particle phases in complex manners. We have studied of the partitioning behavior of both acrylic (AA) and methacrylic (MAA) acids between water and a variety of individual styrene, acrylate, and methacrylate monomers. The distribution coefficients strongly depend upon the hydrogen-bond acceptor characteristics of the organic phase and the pH of the aqueous phase. AA and MAA behave similarly, but AA distributes much less strongly to the organic phase than does MAA. The logs of the distribution coefficients for both vinyl acids correlate linearly with the molar volume of the (meth)acrylate monomers, and these values decrease as the molecular weight of the monomer increases. Vinyl acid distributions to styrene monomer are nearly completely determined by the dimerization of the acids in the monomer phase and, as such, are quite sensitive to the concentration of the acid in the water phase. The effects of ionic strength and temperature are minimal for the usual emulsion polymerization reaction conditions.