Radiation-induced degradation of two alkyl-substituted polysilanes, poly(methyl-n-propylsilane) and poly(di-n-hexylsilane), has been studied in mixed solvents of n-butyl chloride (BuCl) and tetrahydrofuran (THF). The polysilanes readily degraded in BuCl, while the degradation was strongly suppressed by the addition of a small amount of THF. Mechanistic as well as spectroscopic studies revealed that radical cations of the polysilanes play a more important role than the radical anions as intermediates in the radiation-induced degradation.