Copolymerization of methacrylic acid (MAA) and ethyl acrylate (EA) was performed in aqueous medium in the presence of an emulsifier with MAA as the major component (53.7-82.3 mol %). Emulsion stability was studied as a function of concentration and the nature of the emulsifier (ionic or nonionic). Copolymers were characterized using IR and H-1-NMR spectroscopy. C-13-NMR spectra of MAA-EA copolymers have been discussed in terms of their triad monomer sequence distributions. Reactivity ratios of the MAA-EA pair were determined using a nonlinear least-square errors in variables method. Partition of the monomers in aqueous and organic phases was studied using a gas chromatographic technique. A two-loci polymerization mechanism was proposed on the basis of this partition behavior and the calculated composition of the oligomeric radicals formed in the initial stages of polymerization.