Addition of aliphatic polyols to aqueous silicate solutions is shown to yield high concentrations of stable polyolate complexes containing five- or six-coordinated silicon. Coordinating polyols require at least four hydroxy groups, two of which must be in threo configuration, and coordinate to silicon via hydroxy oxygens at chain positions on either side of the threo pair. The remarkable ease by which these simple sugar-like molecules react to form hypervalent silicon complexes in aqueous solution supports a long-standing supposition that such species play a significant role in the biological uptake and transport of silicon and in mineral diagenesis.