The precursors of poly(hydrogen silsesquioxane) (PHSSQ) were synthesized from triethoxysilane (TES) through variations in the pH and molar ratio of water to TES (R-1). The molecular structures of the prepared PHSSQ precursors were controlled by the reaction conditions, including the molecular weight, the content of Si-OH end groups, and the cage/network ratio. The effect of the reaction conditions on the PHSSQ structure was quite different from that previously reported for poly(methyl silsesquioxane) (PMSSQ). The Si-OH content and cage/network ratio of the prepared PHSSQ precursors increased with increasing R-1 and pH. The molecular weight of the prepared PHSSQ precursors first increased with increasing R-1 or pH and then decreased at high values of R-1. These results were explained by the effects of R, and pH on the hydrolysis and condensation reactions. The cage/network ratio of the prepared PHSSQ films decreased. with increasing curing temperatures and resulted in an increasing refractive index or dielectric constant. The steric effect from the side groups yielded PMSSQ films that had a smaller refractive index or dielectric constant than the PHSSQ precursors. PHSSQ films have potential applications as low-dielectric-constantmaterials. (C) 2003 Wiley Periodicals, Inc.