Copolymerization of styrene and 2-hydroxyethyl methacrylate (2-HEMA) was carried out in a microemulsion medium. The composition of the copolymers was estimated using proton H-1-NMR. The reactivity ratios of styrene and 2-HEMA in ternary microemulsions were observed and were considerable different from those reported for solution and bulk polymerization. In monomer pairs with a considerable difference in polarity, partitioning of a monomer between the aqueous phase and the microemulsion droplets develops a concentration gradient, which can be calculated from the distribution coefficient of the monomer between the two phases. This approach has led to more reliable reactivity ratios for the monomers. The study of styrene-2-HEMA copolymerization in a sodium dodecylsulfate-based microemulsion resulted in r(S) = 3.79 and r(H) = 0.17 as apparent reactivity ratios and r(S) = 0.57 and r(H) = 23.24 as true reactivity ratios for styrene and 2-HEMA, respectively.