Solution O-17 NMR spectroscopy was used for structure elucidation of siloxane copolymers with the natural abundance of O-17, i.e. without any enrichment prior to spectroscopy. Homo, co, and terpolymers, as well as linear chains, cyclic oligomers, and graft polymers were investigated. All relevant chemical shifts and corresponding linewidths were reported for siloxane polymers substituted with methyl, phenyl, 3-cyanopropyl, 2-cyanoethyl, 3,3,3-trifluoropropyl, and polyethylene glycol ligands and with the backbone stiffening groups tetramethyl-p-silphenylene, tetramethyl-p,p'-sildiphenylene ether, and m-carborane. An increment system was extended to predict the chemical shifts of substituted siloxane copolymers. O-17 NMR spectroscopy of polysiloxanes provided information concerning their chemical composition, average molecular weight. and microstructure. (c) 2005 Elsevier Ltd. All rights reserved.