Pt-catalyzed hydrosilylation between vinylheptaphenylcyclotetrasiloxane and a series of alpha,omega-bis(hydrido)polydimethylsiloxanes and copoly(methylhydridosiloxane/dimethylsiloxane) was used to prepare chemically modified materials. These modified polymers were characterized by IR, UV, and H-1, C-13, and Si-29 NMR spectroscopy and gel permeation chromatography (GPC). The molecular weights, determined by GPC, UV, and NMR end-group analysis, showed the anticipated increases. The thermal properties of the polymers were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The crystallinity, determined by DSC, was either reduced or completely eliminated for the modified polymers. The thermal stabilities, measured in both air and nitrogen by TGA, were slightly higher than the thermal stability of alpha,omega-bis(trimethylsiloxy)polydimethylsiloxane. Significantly increased bulk viscosities were observed for all the modified polymers. (c) 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3007-3017, 2005.