Silsesquioxanes have been postulated to have complex structures due to varieties of siloxane bond arrangements from intra- or intermolecular condensation reactions. However, most structure types reported in the literature such as ladder or cage structures were proposed on the basis of spectroseopic analysis of the crude product and therefore may have overlooked the structural complexity of these resinous materials. In this study, a (PhSiO3/2)(0.35)(MeSiO3/2)(0.40)(Me(2)ViSiO(1/2))(0.25) ((T0.35T0.40M0.25VI)-T-Ph-M-Me) silsesquioxane was fractionated into 13 fractions by supercritical fluid extraction and analyzed by mass spectrometry (MS), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and size exclusion chromatography (SEC) techniques. Detailed compositional, structural, and molecular weight information on the parent silsesquioxane and individual fractions were obtained. The average composition obtained by NMR varied for the individual fractions, where the Me(2)ViSiO(1/2) content was higher in the low-molecular-weight fractions but remained relatively constant for the high-molecular-weight fractions. On the basis of gas chromatography-mass spectrometry (GC-MS) and electrospray ionization Fourier transform mass spectrometry (ESI FTMS) results for the six lowest molar mass fractions 1-6, the individual species that were identified can be summarized into three general categories: (1) TnM(n+2) (2) T2n+1M2m+1 (odd number of T and M), and (3) T2nM2m (even number of T and M). Species with the TnMn+2 composition have linear or hyperbranched structures while one or more siloxane rings are required for the T2nM2m and T2n+1M2n+1 compositions. Combining Si-29 NMR and FTIR results, we propose that individual molecules in the silsesquioxane adopt structures composed of predominantly fused siloxane rings connected via adjacent (ladderlike) or nonadjacent vertexes (cagelike). Conformational analysis through SEC yielded a Mark-Houwink "a" parameter of 0.64-0.72 in THF for the fractions with M, between 1750 and 26 900 g/mol, which was consistent with a random coil conformation. The flexible nature of the silsesquioxane molecules appeared to confirm the proposed "linear" fused ring structure. Light scattering results for fraction 13 (M, 60 800) and its slightly lower "a" value of 0.502 suggest a higher degree of branching for the structures present in the highest molecular weight fraction.