Silica solubility and speciation at high pressure (P) and temperature (T) conditions are fundamental for the quantitative modeling of the fluid-rock interactions in the Earth’s crust and upper mantle. In subduction zones, alkaline fluids are produced by the interaction between H2O and deep-crustal or subducting rocks, but the chemical properties data of such high P–T alkaline fluids in equilibrium with quartz are still lacking. In the present study, the dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg−1 H2O] Na2CO3 and 0.47 m NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3 and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than that in pure H2O indicate the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1 (silicate center having a single bridging oxygen atom) in the Na2CO3 solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3 and NaOH to fluids, corresponding to an elevation of quartz solubility. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.