Well-defined amphiphilic cubic silsesquioxane-poly(ethylene oxide) (CSSQ-PEO) was prepared from octakis (dimethylsiloxy)octasilsesquioxane (Q(8)M(8)(H)) and allyl-PEO through a hydrosilylation reaction. The structure of CSSQ-PEO was characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). The amphiphilic properties and aggregation process of CSSQ-PEO in aqueous solution were investigated by fluorescence, dynamic and static light scattering (DLS and SLS), and transmission electron microscopy (TEM). The critical aggregation concentration (CAC) determined by fluorescence measurements was found to be 0.28 mg/mL. Combinations of DLS, SLS, and TEM studies showed the existence of core-corona micelle with hydrophobic CSSQ as the core and hydrophilic PEO as the corona in aqueous solution. The observation of two size distribution peaks from DLS measurements revealed the coexistence of small amounts of unassociated unimolecular micelles (similar to 10% of the scattered intensity) together with micellar aggregates when the CSSQ-PEO concentration was <= 2 mg/mL. The hydrodynamic radii (Rh) of unassociated unimolecular micelle and micellar aggregates were found to be 26 and 79 nm, respectively. A large R-g/R-h ratio (1.46) and the extremely small value of average chain density (4 x 10(-4) g/cm(3)) indicate the small hydrophobic CSSQ core was surrounded by the extended PEO coronae. The aggregation number (N-agg) of CSSQ-PEO in aqueous solution was found to be 38 +/- 2 from SLS and 31-40 from TEM, respectively. The long PEO segments act as a spacer between the spherical aggregates, which facilitate the formation of a network-like structure at high concentration.