UV absorption spectra of poly(methylphenylsilane) films were measured at varying temperatures from 25°C to 200°C and a new thermal hysteresis in UV absorption of the polysilane films was observed. Once a polysilane film was annealed above its glass transition temperature, its UV absorption spectrum greatly and irreversibly changed. In other words, the absorption spectrum intrinsic to a particular polysilane film is obtained only after it is annealed above its T_g. This thermal hysteresis is discussed in relation to the changes in the micrtoscopic free spaces and in the hole transport characteristics between the annealed and non-annealed polysilane films. We concluded that after a polysilane film is annealed above its T_g, there occurs an irreversible re-distribution of the σ-conjugated domains in a way as to increase their average size.