A structural investigation of poly(methyl methacrylate) (PMMA) latex particles swollen by styrene is given. The goal of this study is to detect whether the polymer chains will be depleted at; the surface for entropic reasons (wall-repulsion effect) or whether enthalpic effects prevail driving the nonpolar swelling agent into the core of the particles. By use of small-angle X-ray scattering in conjunction with contrast variation, the radial structure of the swollen particles could be determined with high resolution. In the swollen particles a surface layer of about 1 nm thickness has been found in which the polymeric compound is strongly enriched. This core-shell structure becomes more pronounced with increasing amount. of added swelling agent. From this result it is obvious that in the particular case studied here the enthalpic effect dominates : The nonpolar styrene avoids the water interface whereas the more polar polymer is enriched at the surface. This finding, in combination with a previous investigation on the swelling of a PMMA latex with the more polar MMA (J. Bolze, M. Ballauff, Macromolecules 1995, 28, 7429) shows that the radial structure of swollen latex pal-tides may be controlled by entropic and/or enthalpic effects. This is in agreement with a recent theoretical study by Evers et al. (Colloid Surf, A 1994, 90, 135).