Polyethersulfone (PES) - [silicon oxide] hybrids were derived via sol-gel reactions for tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) in dimethylacetamide solutions of the polymer. In one scheme, water was initially present, and condensation reactions between SiOR groups competed with their reactions with - OH groups at PES chain ends. In a second scheme, water addition was delayed; TMOS molecules reacted with chain ends before competing TMOS-TMOS reactions occurred. A third study involved parallel experiments, as follows : 1) introduction of EtOH to PES-TEOS solutions for a time before water addition; and 2) reactions occurring for a time in non-EtOH-containing PES-TEOS solutions before water addition. Infrared (IR) spectroscopy uncovered signatures of Si-O-Si bridges in silicon oxide phases and PES endgroup modifications (Si-O-Ph). Composites prepared according to the latter two schemes contain more Si-O-Ph linkages than those generated via the first. Differential scanning calorimetry showed that T-g can be raised, and thermogravimetric analysis revealed how the PES thermal degradation profile can be modified via these inorganic incorporations. The schemes for late water addition produced composites having increased elongation-to-break and lowered strength relative to unfilled PES.