Thermosoftening phenylpolysiloxane glasses with controlled softening temperature have been obtained by a heat treatment at temperatures close to the decomposition temperature of organic component for several minutes, which we call a rapid heat treatment (RHT). Precursor glasses for RHT should be silanol-free, which were prepared by polycondensation of fully hydrolyzed small oligoiners. The softening temperature (T-S) and weight-average molecular weight (M-W) of the obtained glasses could be controlled by the RHT at 400-500 degrees C for 5 min. The decrease of the molecular weight through the rearrangement of siloxane network was responsible for a decrease of the softening temperature by RHT.