Solvent-corrected reduced isotropic Raman spectra of aqueous boric acid + sodium borate solutions have been obtained from perpendicular and parallel polarization measurements in a novel custom-made titanium flow cell with sapphire windows over the temperature range 25 to 300 degrees C at 20 MPa using the perchlorate anion, ClO4-, as an internal standard. The reduced isotropic spectra of solutions yielded the first reported quantitative speciation results for polyborate ions in equilibrium with boric acid and borate in high-temperature aqueous solutions above 200 degrees C. The spectra obtained from solutions at low sodium/boron ratios, 0 < m(Na)(ST)/m(B)(ST) < 0.254, displayed well-defined bands at 880, 747, 615, and 532 cm(-1), corresponding to the species B(OH)(3) > [B(OH)(4)](-) > [B3O3(OH)(4)](-) >> [B5O6(OH)(4)](-), respectively. The triborate ion, [B3O3(OH)(4)](-), was found to be the major polyborate species in these boric acid-rich solutions in the range 25 to 300 degrees C. Thermodynamic formation constants for the triborate species, [B3O3(OH)(4)](-), calculated from the peak areas, are in agreement with the literature values reported by Mesmer et al at 50, 100, and 200 degrees C to within the combined experimental uncertainties. At 300 degrees C, the value for the formation constant, log K-31,m(b) = 2.259 +/- 0.060, is larger than the value extrapolated from the results of Mesmer et al. by a factor of similar to 3.