Infrared (IR) absorption and Raman spectra are measured for glow‐discharge‐produced, wide‐optical‐gap binary Si:H alloys containing a number of polysilane (SiH2)ngroups (polysilane alloys). The highly resolved IR stretching band revealed double peaks, at 2100 and 2122 cm−1, which correspond to an exterior SiH2and an interior SiH2, respectively, in a 3/4 Si(SiH2)nSi 3/4 group. This result provides evidence for the existence of (SiH2)n(n≥3) groups. Nonlinearity of the (SiH2)nwagging absorption strength with the number of SiH2units in the alloy is observed in the IR spectra. This is related to a change in the polymerization degree of a (SiH2)nchain. Raman scattering intensity in the 50–400 cm−1range is lower than that for conventional hydrogenated amorphous silicon alloys, suggesting a reduction in the number of Si loops arising from (SiH2)nchain formations. The change in the polymerization degree as a function of substrate temperature during deposition and the annealing temperature are also discussed.