Electronic absorption spectra of radical ions, both cations and anions, of various molecular weight poly(methyl-n-propylsilane)s were measured in rigid matrices at 77. The radical ions were produced by gamma-irradiation. The spectra had two peaks in UV and near infrared regions, and the maxima in both UV and near infrared regions shifted to longer wavelengths with increasing chain length until about 16 monomer units and showed saturation above this length. Excellent inverse correlation was obtained between the transition energy and the chain length; this reflects increasing electronic delocalization with increasing number of silicon atoms in the radical anions and cations.