The aggregate formation in aqueous solutions of Brij-35 (C12EO28) nonionic surfactant has been studied in the concentration range from 5 to 200 g/L at 20, 40, and 60 degrees C. The structure of micelles was modeled by a spherical core with tethered polymer chains to the surface, while the intermicellar interaction was described by a hard sphere structure factor. The mean aggregation number was found to increase substantially with both concentration and temperature from 34 (5 g/L, 20 degrees C) to 64 (200 g/L, 60 degrees C). The average core radii obtained are between 18 and 19 Angstrom. On the basis of the above two parameters, 35-50% of the core volume is occupied by hydrated poly(ethylene oxide) (PEO) chains. The radius of gyration of tethered chains, R-gc, is in agreement with the theoretical value for Gaussian conformation (11.6 Angstrom) at the smaller concentrations measured but increases significantly with increasing concentration. It reveals considerable stretching of the chains that is attributed to sterical constraints. At higher concentrations R-gc decreases with temperature similarly to the interaction radius. This fact is interpreted as a change in the conformational equilibrium of PEO chains.