Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase is a viscous liquid, gel, or precipitate. In recent years, this phenomenon has been exploited to form gel-like particles, ranging from approximately 100 to 4000 μ in diameter, whose stability depends on equilibrium phase behavior. As the sample composition is varied, these particles either remain stable (in a two-phase mixture) or dissolve over time. Here, we present the formation of reversibly swelling gel particles from mixtures of N,N,N-trimethylarnrnonium- derivatized hydroxyethyl cellulose (JR-400) and sodium dodecyl sulfate (SDS), whose swelling is controlled by the ambient solution conditions. The effects of cross-linking density and surfactant concentration are investigated by gravimetry and confocal microscopy. The resulting particles have a core/shell morphology and undergo reversible swelling/collapse transitions which, depending on the cross-link density, can be either gradual or abrupt with changing SDS concentration.