Study shows that these five polyborate species B(OH)₃, 33c37de70defb098079c3c1a0e05ba">${\text{B}\left(\text{OH}\right)}_4^{\mathrm{-}}$, ${\text{B}}_3{\text{O}}_3{\left(\text{OH}\right)}_4^{\mathrm{-}}$, c8022ba134561993d91e">${\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4^{2\mathrm{-}}$ and ${\text{B}}_5{\text{O}}_6{\left(\text{OH}\right)}_4^{\mathrm{-}}$ mainly existed in aqueous RbB₅O₈ solution.

Furthermore, the higher temperature is beneficial to formation of B(OH)₃ and 33c37de70defb098079c3c1a0e05ba">${\text{B}\left(\text{OH}\right)}_4^{\mathrm{-}}$, while the lower temperature is conductive to formation of ${\text{B}}_3{\text{O}}_3{\left(\text{OH}\right)}_4^{\mathrm{-}}$, c8022ba134561993d91e">${\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4^{2\mathrm{-}}$ and ${\text{B}}_5{\text{O}}_6{\left(\text{OH}\right)}_4^{\mathrm{-}}$.

The average interatomic distance and coordination number are 2.93 ± 0.004 Å and 7.7 ± 0.9 in first hydration shell around rubidium ion by extended X-ray absorption fine structure, and the second hydration shell of Rb⁺ are not found in aqueous RbB₅O₈ solutions.