Species dissolved in H2 fluid were investigated in a SiO2–H2 system. Raman and infrared (IR) spectra were measured at high pressure and room temperature after heating experiments were conducted at two pressure and temperature conditions: 2.0 GPa, 1700 K and 3.0 GPa, 1500 K. With the dissolution of quartz, a SiH vibration mode assignable to SiH4 was detected from Raman spectra of the fluid phase. Furthermore, an OH vibration mode was observed at 3260 cm−1 from the IR spectra at 3.0 GPa. With decreasing pressure, the OH vibration frequencies observed between 3.0 and 2.1 GPa correspond to that of ice VII, and those observed at 1.4 and 1.1 GPa correspond to that of ice VI. These results indicate that the chemical reaction between dissolved SiO2 components and H2 fluid caused the formation of H2O and SiH4, which was contrastive to that observed in SiO2–H2O fluid. Results imply that a part of H2 is oxidized to form H2O when SiO2 components of mantle minerals dissolve in H2 fluid, even in an iron-free system.