The monomer chosen for the synthesis of this polymer, diethoxy-p-fluorophenylmethylsilane, was prepared by the Grignard reaction of methyltriethoxysilane withp-fluorophenyl magnesium bromide. The structure and purity of the monomer were characterized by gas chromatography and mass, infrared, and proton NMR spectroscopy. It was hydrolyzed in aqueous HCI solution and then polymerized by dehydration and ring-chain equilibration. The resulting polymer, poly(p-fluorophenylmethylsiloxane) [-Si(p-C6H4F)(CH3)O-]x was characterized by infrared spectroscopy, and proton and fluorine NMR spectroscopy. Its number-averagemolecular weight. approximately 7 × 104g mol−1, may be high enough for evaluation of this material as a gas-separation membrane.