The anionic solid-state polymerization of triclinic crystals of hexaphenyl-cyclotrisiloxane (HPhTS) initiated by KOH and potassium oligo (methylphenylsiloxane) has been studied. It was found that this reaction can yield high molecular weight poly(diphenylsiloxane) (PDPhS) with a specific viscosity up to 5 (1 wt % diphenyloxide solution at 145 degrees C). The main features of the process are as follows : (a) this is a heterogeneous reaction that proceeds inward from the surface of HPhTS crystals; (b) the crystalline polymer is obtained from the crystalline trimer; (c) OPhTS simultaneously forms along with the polymer; (d) the specific viscosity of the resulting polymer remains constant or decreases with polymerization time and, consequently, with the conversion of HPhTS; and (e) the crystallinity of polymerized PDPhS samples depends inversely on its specific viscosity. Together, these features suggest that polymerization and crystallization proceed successively. The morphologies of the resulting PDPhS phase revealed by means of scanning electron microscopy are consistent with this mechanism.