The preparation of reversibly softening polymers and copolymers of phenylsilsesquioxane in a one-step synthesis has been studied. The synthetic methods are based on hydrolytic polycondensation of PhSiCl3 or copolycondensation of PhSiCl3 with Ph2SiCl2 using moistened or hydrated salts: (1) moistened Na2CO3, K2CO3, (2) Na2B4O7·10H2O, (3) Na3PO4·12H2O. Syntheses (1) and (2) gave products as reversibly softening and soluble materials, whereas synthesis (3) produced partially cross-linked resins from which a soluble material might be extracted. Polycondensation using borax gave polymers and copolymers with incorporated boron atoms. The polysilsesquioxanes were characterized by size exclusion chromatography (SEC), 1H and 29Si NMR, IR spectroscopy, and elemental analysis. Thermal properties were studied by DSC and thermogravimetry.