Amino-terminated three-armed polystyrene stars of narrow molecular weight distribution were prepared by anionic polymerization. The association of these star polymers with a low molecular weight dinitrophenol compound was studied by means of light scattering and ultraviolet-visible spectroscopy in the good solvent 1,4-dioxane and in the Theta solvent diethyl malonate. The concentration dependence of the scattered intensity was used to derive the increase in weight average molecular weights of the associates M(w)(c) where a correction was applied that allowed the separation of the influence of repulsion forces. The experimental molecular weights were found to be smaller than predicted by the Flory-Stockmayer branching theory. Deviations are stronger for star polymers of low molecular weight, which is explained by intramolecular complex formation, No substantial difference was detected between the association in good and Theta solvents. Gel formation did not occur, and reasons are discussed.