The Si-based structural defects in poly(methylphenylsilylene) backbones were detected as two new Si-29 NMR signals and as two new Si-Si vibrational IR bands. The defects, consisting of an organosilyne unit and about three methylphenylsilylene monomer units near the branch, were presumed to be in a fairly lengthened Si-Si bonding state. The empirical linear relationship between the relative intensity of broad photoluminescence and the defect density predicts the disappearance of the broad luminescence for a defect density of less than 1 %.