The kinetics and mechanism for the process of radicals entering latex particles from the aqueous phase ("radical entry") in electrosterically stabilized emulsion polymerization particles were studied. Polystyrene particles stabilized with differing lengths of poly(acrylic acid) chains bound to the surface were synthesized using RAFT-controlled radical polymerization techniques to ensure that the hydrophilic block was of low polydispersity; these latexes had been studied previously to examine the exit rate coefficient (k) in such systems via γ-relaxation experiments. Particles stabilized with poly(acrylic acid) chains were shown to have a significantly lower average number of radicals per particle (n) (and hence reaction rate) than that of equivalent systems stabilized with a conventional surfactant when used in seeded chemically initiated dilatometry experiments with styrene. Assuming that these emulsion systems obey second-order loss kinetics (i.e., an exited radical will re-enter another particle to either propagate or terminate, a phenomenon well accepted for styrene emulsion systems), the calculated entry rate coefficient (ρ) was significantly lower than that predicted by the "control by aqueous phase growth" mechanism for radical entry. Excellent agreement with the accepted entry model was found to occur when radical loss was assumed to be a first-order process with respect to n̄ (i.e., an exited radical is terminated in the aqueous phase) or if significant amounts of aqueous phase termination is introduced into the evolution equations of initiator-derived oligomers. These results may be explained by rapid transfer of radical activity to a poly(acrylic acid) chain on the surface, forming "midchain radicals" that increase the likelihood of termination events in the poly(acrylic acid) surface layer.