A series of modified natural rubber latexes (NELs) grafted with poly(methyl methacrylate) (PMMA) were prepared by seeded emulsion polymerization with NRL as the seed polymer. Two different redox systems, cumene hydroperoxide (CHP)/tetraethylene pentamine (TEPA) and tert-butyl hydroperoxide (t-BHP)/TEPA, were used to initiate polymerization, and phase mixing was promoted by the addition of vinyl neo-decanoate (VneoD). The CHP/TEPA system was more efficient than t-BHP/TEPA for the grafting of secondary polymers in modified natural rubber (NR). The enhanced phase mixing in the presence of VneoD was attributed to the solubility parameter of the VneoD-rich methyl methacrylate-VneoD copolymer formed late in the reaction, lying between that of PMMA and NR, and the extent to which this polymer was grafted to the NR backbone. The viscoelastic properties of the polymers were investigated as a function of composition, temperature, and frequency:, changes in viscoelastic behavior consistent with the presence of a high-T-g PMMA phase (where T-g is the glass-transition temperature) were observed. This suggested a degree of phase mixing that increased with increasing VneoD content and increasing flux of oxygen-centered radicals within the NR particles. More phase mixing resulted in poorer film formation, which was consistent with the localization of a high-Tg secondary polymer phase near the particle surface. The apparent concentration of PMMA near the surface of the particles was also observed with transmission electron microscopy. The localization of PMMA near the particle surfaces was consistent with the presumed locus of radical generation in these systems: the redox couple used to initiate the polymerization consisted of an oil-soluble hydroperoxide and a water-soluble amine that reacted predominantly at the water/particle interface. The viscoelastic properties of the modified NFLs that were prepared suggest that these synthetic procedures provide a means of controlling phase mixing and branching, such as for improving the suitability of these modified rubbers in pressure-sensitive-adhesive formulations.