Heteropolyanions, [GaNb₁₈O₅₄]¹⁵⁻ and [AlNb₁₈O₅₄]¹⁵⁻, have been synthesized and characterized. These represent the first examples of introducing group III elements into the heteropolyniobate family; achieved via compatible alkaline aqueous chemistry of group IIIA monomers and Nb-polyoxoanions. The open structure of the polyniobate results in expanded coordination of the tetrahedral Ga in aqueous medium.