Dynamic light scattering and ultracentrifugation studies have shown that, in aqueous solutions of some anionic surfactants such as sodium dodecyl sulfate (SDS), poly(vinyl acetate) (PVAc) latex particles swell up to 60 times their original volume. The process is reversible on dilution. This unexpectedly large swelling always commences at a surfactant concentration below the critical micelle concentration, e.g., at 4.5 mM for SDS. The swelling rate is best expressed by a Kohlrausch-Williams-Watts dispersive kinetics expression-initially very fast and then becoming progressively and continuously slower. Swelling is inferred to be due to the formation throughout the particle of polymer-micelle complexes, the structure of which is similar to those reported in systems containing certain water-soluble synthetic polymers and surfactants. The swelling is reversible and on dilution the particles contract rapidly to slightly less than their original size. The difference is due to the escape of some lower molecular weight material from the particles. The reversibility shows that a degree of morphological integrity of the particle is preserved on swelling. This is attributed to the existence of microdomains of intrinsic entanglements probably formed during the original emulsion polymerization process.