A soluble polysilsesquioxane (PSQ) containing phosphonic acid side-chain groups (PSQ-PO3H2) was successfully prepared by hydrolytic polycondensation of 2-(diethoxyphosphoryl) ethyltriethoxysilane using concentrated hydrochloric acid as a catalyst. The Si-29 NMR spectrum of PSQ-PO3H2 indicated two broad T-3 and T-2 signals with an integrated ratio of ca. 0.88: 0.12. The weight-average molecular weight of PSQ-PO3H2, as determined via the Debye-plot method using the static light scattering in dimethyl sulfoxide, was ca. 1.8 x 10(4). Furthermore, a soluble PSQ containing phosphonate side-chain groups (PSQPO(3)HK) was obtained by treatment of PSQ-PO3H2 with aqueous potassium hydroxide. The X-ray diffraction pattern of the PSQ-PO3HK cast film showed diffraction peaks derived from a hexagonal phase formed by regular stacking of one-dimensional rod-like PSQs. According to these results, the resulting PSQs would have ladder-like structures. In addition, PSQ-PO3H2 exhibited superior thermal stability (T-d10 > 460 degrees C) and relatively high proton conductivity (6.3 x 10(-2) S cm(-1) at 80 degrees C and 90% relative humidity). (C) 2017 Elsevier Ltd. All rights reserved.