The rate coefficient for radical entry into latex particles in emulsion polymerizations is measured for styrene systems in which the entering species are anionic (from persulfate) and cationic (from 2,2'-azobis(2-methylpropionamidine) dihydrochloride, or V-50). These entry rate coefficients p are obtained by measuring rates in seeded emulsion polymerizations where the seeds have either cationic or anionic groups on the surface; "zero-one" conditions are employed, because these offer the advantage that particle size is sufficiently small (approximate to70 nm diameter) that intraparticle termination is not rate-determining. Data comprise steady-state rates with chemical initiator, combined with loss rates obtained using gamma-radiolysis initiation and following the relaxation behavior after removal from the radiation source. Values for p as a function of initiator concentration can be meaningfully compared for different initiators through the dependence of initiator efficiency f(entry) on primary radical generation rate (radical flux). For the anionic latex, this dependence is seen to differ depending on the nature of the initiator used. This may be explained by the entry model [Maxwell, I. A.; Morrison, B. R.; Napper, D. H.; Gilbert, R. G. Macromolecules 1991, 24, 1629] wherein the rate-determining steps in entry are assumed to be only aqueous-phase propagation and termination to form surface-active z-meric oligomeric radicals; entry is solely by z-mers, for which actual entry into the particle is so fast as not to be rate-determining. The cationic latex shows a high rate of spontaneous initiation, which can be explained in terms of amidino radical chemistry; this can be reduced by heat treatment. Accurate f(entry) values are obtained using the heat-treated cationically stabilized latex for seeded studies. The f(entry) data are also consistent with the model for both cationic and anionic species entering the cationic latex. Values of z so inferred (approximate to2 for persulfate, approximate to1 for amidinium) can be understood in terms of the hydrophobic free energy of these species. The results refute alternative models in the literature that suppose entry is controlled by double-layer (colloidal) interactions, surfactant displacement, or diffusion control.