The monomer reactivity ratios of styrene (r(s)) and methyl methacrylate (r(M)) copolymerization in ternary oil-in-water microemulsions were determined from the H-1 NMR spectra. The obtained values of r(s) = 0.74 +/- 0.09 and r(M) = 0.38 +/- 0.04 are different from those determined by homogeneous polymerizations. The results are discussed in terms of the partitioning of monomers in the microemulsions and the copolymerization loci. Information about the microstructures of copolymers was derived from the analysis of the methoxyl region of the H-1 NMR spectra and the alpha-methyl region as well as the aromatic C1 region of the C-13 NMR spectre. The experimental results on the copolymer microstructures are in good agreement with the calculated values using the newly determined r(s) and r(M). The glass transition-temperatures (T-g) of the copolymers were found to decrease linearly with increasing mole fraction of styrene in the copolymers.