The crystal structure of metal borates has been extensively inves-tigated by X-ray and neutron diffraction,but the structure of aqueous polyb-orate solutions are still largely unknown. Over the last decade,our group has focused on studying the structure of complex aqueous polyborate solutions of Li,Na,K,Rb,Cs,and Mg using synchrotron radiation X-ray scattering (XRS),EXAFS,Raman,NMR,and DFT,as well as determining the densi-ty,conductivity and pH of such solutions. Polyborate species distributions were calculated using pH measurements,and the main species in the solu-tion have been confirmed by NMR and Raman spectra. For alkali-metal metaborates,the dominant species is always B(OH)-4 in a wide range of concentration,while the presence of others species is negligible. For alkali metal tetraborates,when concentration is in the extreme low range,only B(OH) 3 and B (OH)-4 are present in these solutions. As the total boron concentration increases, B(OH) 3 and B(OH)-4 polycondensate to form more complex oligomers. Of them,while B 4 O5 (OH)2 -4 in the tetraborate solutions is the main species,B(OH) 3 ,B(OH)-4 ,and B 3 O3 (OH)-4 are minor spe-cies,and B 3 O3 (OH)2 -5 and B5 O6 (OH)-4 are present only in negligible amounts. As solution continues to concentrate,B 4 O5 (OH)2 -4 eventually becomes the dominant species,which is consistent with the congruent compound M2 B4 O7·nH 2 O (M = Li,Na,K,Rb,Cs)in the system M 2 O - B2 O3 - H2 O (M =Li,Na,K,Rb,Cs). For alkali metal pentaborates,B(OH) 3 and B(OH)-4 are the main species at low concentrations. The species distribution,Raman and NMR spectroscopy results verified that the domi-nant species in concentrated pentaborate solutions with Li and Na is pentaborate B5 O6 OH)-4 ,but it is surprising that the main species with K,Rb,and Cs is always the triborate monoanion B 3 O3 (OH)-4 . Al-though all M[B 5 O6 OH) 4 ]·nH 2 O (M = Li,Na,K,Rb,Cs)are congruent compounds in the system M 2 O - B2 O3 - H2 O (M = Li,Na,K,Rb,Cs),the main species in aqueous solutions are quite different be-cause of various cation hydration distance (d),hydration number (CN),and configuration,especially charge (Z). For bivalent Mg2 +,three borate minerals,namely,Inderite (2MgO·3B 2 O3·15H 2 O),Hun-gchaoite (MgO·2B 2 O3·9H 2 O),and Mcallisterite (MgO·3B 2 O3·7. 5H 2 O),exist in the system MgO -B2 O3 - H2 O at 298K. Inderite is a congruent compound,but Hungchaoite and Mcallisterite are incon-gruent compounds. The species distribution and Raman spectra demonstrat that the predominant species in all the solutions with magnesium borates is bivalent triborate B3 O3 (OH)2 -5 ,while the subordinate species are B(OH)-4 at a low B2 O3 / MgO ratio and B(OH) 3 at a high B2 O3 / MgO ratio,and the other anions are negligible,as the high Z of the borate anion must match that of Mg2 + . The disappearance of divalent tetraborate and hexaborate is in agreement with its incongruent nature. The DFT and XRS re-sults showed that tetrahedral Li(H 2 O)+4 (d = 0. 20 nm,CN = 4),octahedral Na(H 2 O) 6+(d = 0. 236 nm,CN = 6),and Mg(H 2 O)2 +6 (d = 0. 210 nm,CN = 6)in the first hydration shell belong to Platonic polyhedra. However,K(H 2 O)+8 (d = 0. 28 nm,CN = 8,XRS and DFT),Rb(H 2 O)+8 (d = 0. 293 nm, CN = 7. 7 - 8. 2,EXAFS),and Cs(H 2 O)+8 (d = 0. 320 - 0. 326 nm,CN = 7. 6 - 7. 9,EXAFS)are in-clined Voronoi polyhedra. Therefore,the effects of cation Z and d on the structure of polyborates in a-queous solutions are deterministic,while the effect of hydrated-cation symmetry is secondary. Their hy-drolysis order was:Mg > Li > Na > K > Rb > Cs,in step with hydration power. Among them,Mg2 + and Li + have a strong tendency towards hydrolysis,but Na + scarcely hydrolyzes,especially Rb + and Cs +have a little protonation. X-ray scattering of aqueous alkaline sodium borohydride solutions confirmed that dihydrogen bonds exist in the aqueous solution. The four distinct features of dihydrogen bonds in a-queous solution——unidirectionality,divaricativity,multicentricity,and multidentativity——have been also described here in brief.