The molecular structure of the monosilicon analog of poly(ethylene), poly(silylenemethylene), CH3[SiH2CH2]nSiH3, or poly(silaethylene) (PSE), was characterized by means of high-resolution H-1, C-13, and Si-29 NMR spectroscopy. The average molecular weight of this polymer was also determined from the NMR spectra by integration of the -SiH3 and -CH3 end groups, relative to the main-chain -SiH2CH2- units. All of the peaks in the NMR spectra were assigned with the help of a model compound, CH3SiH2CH2SiH3, and PSE with added SiCH2SiH3 or SiCH3 side groups. In addition to the main-chain -SiH2CH2- species, the -CH3 and -SiH3 end groups and a small concentration (ca. 1 per 200 Si atoms in the polymer chain) of -SiHCH3- branch sites were evidenced in the NMR spectra of the "parent" PSE. These -SiHCH3- groups were concluded to arise from a small amount of dimethyldichlorosilane in the methyltrichlorosilane starting material used in the synthesis of the 1,1,3,3-tetrachloro-1,3-disilacyclobutane monomer.